[mips] Removing __SIZEOF_INT128__ macro for MIPS64

This is a temporary workaround while MIPS64 has not yet fully supported
128-bit integers. But declaration of int128 type is necessary even though
`__SIZEOF_INT128__` is undefined because c++ standard header files like
`limits` throw error message if `__int128` is not available.

Patch by Sagar Thakur.

Differential Revision: http://reviews.llvm.org/D6402

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223927 91177308-0d34-0410-b5e6-96231b3b80d8