Update the libc BUILD.bazel file with selects for Windows builds. (#131172)

The Windows toolchain needs to build libc targets in order to support
libc++. Currently, some targets fail to resolve, due to non-exhaustive
select statements, not accounting for Windows builds.

This change adds clauses to select statements so that Windows builds can
proceed.

Additionally, `__support_osutil_exit`, is configured to pass nothing to
`srcs` for non-Linux builds. `__support_osutil_exit `is unconditionally
included in the transitive dependencies of `extern_libc_shared`.

---------

Co-authored-by: Brian Ledger <brianpl@google.com>

Author: bashi-bazouk

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -1319,6 +1319,7 @@ libc_support_library(
             "src/__support/OSUtil/linux/aarch64/syscall.h",
             "src/__support/OSUtil/linux/x86_64/syscall.h",
         ],
+	"@platforms//os:windows": [],
     }),
     deps = [
         ":__support_common",
@@ -1360,6 +1361,7 @@ libc_support_library(
     textual_hdrs = select({
         "@platforms//os:macos": ["src/__support/OSUtil/darwin/io.h"],
         "@platforms//os:linux": ["src/__support/OSUtil/linux/io.h"],
+	"@platforms//os:windows": ["src/__support/OSUtil/windows/io.h"],
     }),
     deps = [
         ":__support_common",