Fix mips and mips64 builds.

MIPS triggers some warnings in libc++ such as:
external/libcxx/src/thread.cpp:105:9: error: hardware_concurrency not yet implemented [-Werror,-W#warnings]

Test: external/clang/build.py
Change-Id: I5ccb4ffe902e29cf74de220f9610da13b1070dc1
1 file changed