mk: Fix defs for i686-w64-mingw32
Remove build system support for i686-pc-mingw32 triple in favor if i686-w64-mingw32
mk: Convert to LLVM-style canonical windows triples

i686-w64-mingw32 -> i686-pc-windows-gnu
x86_64-w64-mingw32 -> x86_64-pc-windows-gnu