[engine,build] Fix Fuchsia RBE build (flutter#177865)

As in the Dart build here:


https://github.com/dart-lang/sdk/blame/fc8b3d79277c55f613423e125f49c80643d2498a/build/toolchain/fuchsia/fuchsia_toolchain.gni#L24

For unknown reasons, the `__config_site` file has to be explicitly
listed as an input to the build to ensure it is available in the RBE
workers.

This PR also cleans up unnecessary flags from the Fuchsia toolchain
definition. The target triple and sysroot flags are already injected
into the compiler flags by the Fuchsia build configs.

linux_fuchsia presub ran in 16 minutes.

Author: zanderso

engine/src/build/toolchain/fuchsia/BUILD.gn

@@ -7,9 +7,25 @@ import("//build/toolchain/clang.gni")
 import("//build/toolchain/rbe.gni")
 import("//build/toolchain/toolchain.gni")
 
+if (target_cpu == "x64") {
+  target_triple = "x86_64-unknown-fuchsia"
+} else if (target_cpu == "arm64") {
+  target_triple = "aarch64-unknown-fuchsia"
+} else {
+  assert(false, "Unsupported target_cpu: $target_cpu")
+}
+
 if (use_rbe) {
-  compiler_args =
-      rewrapper_command + [ "--labels=type=compile,compiler=clang,lang=cpp " ]
+  remote_wrapper =
+      rebase_path("//flutter/build/rbe/remote_wrapper_linux.sh", root_build_dir)
+  local_wrapper =
+      rebase_path("//flutter/build/rbe/local_wrapper.sh", root_build_dir)
+  compiler_args = rewrapper_command + [
+                    "--remote_wrapper=$remote_wrapper",
+                    "--local_wrapper=$local_wrapper",
+                    "--labels=type=compile,compiler=clang,lang=cpp",
+                    "--inputs=$buildtools_path/${host_os}-${host_cpu}/clang/include/$target_triple/c++/v1/__config_site",
+                  ]
   compiler_prefix = string_join(" ", compiler_args)
   link_prefix = ""
 } else if (use_ccache) {
@@ -26,8 +42,7 @@ toolchain("fuchsia") {
          "We currently only support 'x64' and 'arm64' targets for fuchsia.")
   toolchain_bin =
       rebase_path("$buildtools_path/${host_os}-${host_cpu}/clang/bin",
-                  root_out_dir)
-  fuchsia_sdk = rebase_path("//third_party/fuchsia-sdk/sdk", root_out_dir)
+                  root_build_dir)
 
   # We can't do string interpolation ($ in strings) on things with dots in
   # them. To allow us to use $cc below, for example, we create copies of
@@ -40,14 +55,6 @@ toolchain("fuchsia") {
   nm = "${toolchain_bin}/llvm-nm"
   strip = "${toolchain_bin}/llvm-strip"
 
-  if (target_cpu == "arm64") {
-    target_triple_flags = "--target=aarch64-fuchsia"
-  } else {
-    target_triple_flags = "--target=x86_64-fuchsia"
-  }
-
-  sysroot_flags = "--sysroot ${fuchsia_sdk}/arch/${target_cpu}/sysroot"
-
   # These library switches can apply to all tools below.
   lib_switch = "-l"
   lib_dir_switch = "-L"
@@ -59,7 +66,7 @@ toolchain("fuchsia") {
 
   tool("cc") {
     depfile = "{{output}}.d"
-    command = "$compiler_prefix $cc -MD -MF $depfile $target_triple_flags $sysroot_flags $lto_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+    command = "$compiler_prefix $cc -MMD -MF $depfile $lto_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "CC {{output}}"
     outputs =
@@ -68,7 +75,7 @@ toolchain("fuchsia") {
 
   tool("cxx") {
     depfile = "{{output}}.d"
-    command = "$compiler_prefix $cxx -MD -MF $depfile $target_triple_flags $sysroot_flags $lto_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
+    command = "$compiler_prefix $cxx -MMD -MF $depfile $lto_flags {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "CXX {{output}}"
     outputs =
@@ -77,7 +84,7 @@ toolchain("fuchsia") {
 
   tool("asm") {
     depfile = "{{output}}.d"
-    command = "$cc -MD -MF $depfile $target_triple_flags $sysroot_flags $lto_flags {{defines}} {{include_dirs}} {{asmflags}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+    command = "$cc -MMD -MF $depfile $lto_flags {{defines}} {{include_dirs}} {{asmflags}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "ASM {{output}}"
     outputs =
@@ -109,7 +116,7 @@ toolchain("fuchsia") {
     # existing .TOC file, overwrite it, otherwise, don't change it.
     tocfile = sofile + ".TOC"
     temporary_tocname = sofile + ".tmp"
-    link_command = "$link_prefix $ld $target_triple_flags $sysroot_flags $lto_flags -shared {{ldflags}} -o $unstripped_sofile -Wl,--build-id=sha1 -Wl,-soname=$soname @$rspfile"
+    link_command = "$link_prefix $ld $lto_flags -shared {{ldflags}} -o $unstripped_sofile -Wl,--build-id=sha1 -Wl,-soname=$soname @$rspfile"
     toc_command = "{ $readelf -d $unstripped_sofile | grep SONAME ; $nm -gD -f p $unstripped_sofile | cut -f1-2 -d' '; } > $temporary_tocname"
     replace_command = "if ! cmp -s $temporary_tocname $tocfile; then mv $temporary_tocname $tocfile; fi"
     strip_command = "$strip -o $sofile $unstripped_sofile"
@@ -146,7 +153,7 @@ toolchain("fuchsia") {
     outfile = "{{root_out_dir}}/$exename"
     rspfile = "$outfile.rsp"
     unstripped_outfile = "{{root_out_dir}}/exe.unstripped/$exename"
-    command = "$link_prefix $ld $target_triple_flags $sysroot_flags $lto_flags {{ldflags}} -o $unstripped_outfile -Wl,--build-id=sha1 -Wl,--start-group @$rspfile {{solibs}} -Wl,--end-group {{libs}} && ${strip} -o $outfile $unstripped_outfile"
+    command = "$link_prefix $ld $lto_flags {{ldflags}} -o $unstripped_outfile -Wl,--build-id=sha1 -Wl,--start-group @$rspfile {{solibs}} -Wl,--end-group {{libs}} && ${strip} -o $outfile $unstripped_outfile"
     description = "LINK $outfile"
     rspfile_content = "{{inputs}}"
     outputs = [