Change -mno-mmx to be more compatible with gcc. Specifically, -mno-mmx should not imply -mno-sse.

Note that because we don't usually touch the MMX registers anyway, all -mno-mmx needs to do is tweak the x86-32 calling convention a little for vectors that look like MMX vectors, and prevent the definition of __MMX__.

clang doesn't actually stop the user from using MMX inline asm operands or MMX builtins in -mno-mmx mode; as a QOI issue, it would be nice to diagnose, but I doubt it really matters much.

<rdar://problem/9694837>

llvm-svn: 134770