[mips] Fix sem_init_glibc test for MIPS.

glibc changed the implementation of semaphores for glibc 2.21 requiring
some target specific changes for this compiler-rt test. Modify the test
to cope with MIPS64 and do some future/correctness work by tying the
define for MIPS64 to exactly the define of __HAVE_64B_ATOMICS in glibc.

Contributions from Nitesh Jain.

Reviewers: eugenis

Differential Revision: https://reviews.llvm.org/D37829

llvm-svn: 313248