Do not define CRT_HAS_128BIT for MIPS64

Do not define it for MIPS64 until its backend supports it.
See the bug report [1] for more information.

[1] http://llvm.org/bugs/show_bug.cgi?id=20098

Differential Revision: http://reviews.llvm.org/D6703

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224488 91177308-0d34-0410-b5e6-96231b3b80d8