mk: Fix defs for i686-w64-mingw32
Remove build system support for i686-pc-mingw32 triple in favor if i686-w64-mingw32
auto merge of #17200 : brson/rust/wintrip-w64, r=cmr

This updates our build system to prefer `i686-w64-mingw32` as the 32-bit windows triple instead of `i686-pc-mingw32`. This is an interim step to make the build artifacts consistent until https://github.com/rust-lang/rust/issues/15717 is done.