scons: Disable SSE intrinsics on MinGW.

scons: Disable SSE intrinsics on MinGW.

-mstackrealign causes stack corruption on MinGW. And without it the ability
to use SSE instrinsics goes down the drain. Even if we use
__attribute__((force_align_arg_pointer)) for the functions we explicitly
use SSE instrinsics, the SSE code automatically generated by gcc will
cause assertion failures. What a nightmare.

Thankfully LLVM gets this right, so all runtime generated SSE code just
works. rtasm code doesn't assume 16byte alignment. Therefore the bulk of
our performance sensitive code is not affected by this.

Still, intrinsics can be convenient, and it would be nice
to get this working again some day, sp will try to get a reduced test
case.