[mips][compiler-rt] Provide 64bit atomic add and sub

r318733 introduced a build failure for native MIPS32 systems for xray due
to the lack of __sync_fetch_and_add / __syn_fetch_and_sub support. This patch
extends the existing support providing atomics so that xray can be
successfully built.

Reviewers: atanasyan, dberris

Differential Revision: https://reviews.llvm.org/D40385

llvm-svn: 321260