Support 16-byte stack alignment on 32-bit Windows for MinGW/Cygwin?

For 32-bit Mac we've always needed 16-byte stack alignment, and for Linux we recently moved to 16-byte to match the ABI change there (#847, #3966, #4200). However, the 32-bit Windows ABI only mandates 4-byte alignment, so we only supply that today. The problem comes up with binaries *not* built by MSVC: binaries built by gcc or clang through MinGW or Cygwin or whatnot that assume 16-byte alignment. Those binaries have their own problems if they pass callbacks to Windows libraries, so maybe all of them use -mstackrealign already? In which case we don't have to do anything.

Filing this just to document the issues. Not planning to proactively do anything in the short term.