How about remove MINGW32/UCRT64/MINGW64 and introduce CLANG32 back.

[lygstate]

I think now ucrt are have much better Unicode support, we can drop the old
msvcrt.dll support, and for Windows 10 and upper always have ucrt
for Windows XP to 8 we can ship ucrt along with the application, so that's not a issue at all.

The default toolchain use llvm and fallback to gcc when needed.
There is not need create two set of that.

[lygstate]

UCRT64 may preserved for gcc libstdc++ if needed.

[taozuhong]

UCRT64 must be preserved. MINGW32/CLANG32 should keep be dropped.