Windows: Update the build script and README-Windows.txt.

The 32-bit build is now for i686 or newer because the
prebuilt MinGW-w64 toolchains include i686 code in the
executables even if one uses -march=i486.

The build script builds 32-bit SSE2 enabled version too.
Run-time detection of SSE2 support would be nice (on any OS)
but it's not implemented in XZ Utils yet.
2 files changed