Enable __float128 for mingw for GCC compatibility and define __SIZEOF_FLOAT128__ on x86

GCC defines __FLOAT128__ on Power and __SIZEOF_FLOAT128__ on x86. We're
just following the inconsistency for now so users have some way to test.

Effectively merges this patch as requested by Martell Malone:
https://github.com/Alexpux/MINGW-packages/blob/master/mingw-w64-clang/0107-enable-__float128-for-X86-MinGW.patch

llvm-svn: 304012