Fix typos in compiler-rt/lib/builtins/atomic.c

Summary:
This patch fixes typos in file compiler-rt/lib/builtins/atomic.c.

Reviewers: jasonliu, hubert.reinterpretcast, jfb

Reviewed By: jfb

Subscribers: t.p.northover, theraven, dberris, jfb, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D59228

Patch by Xing Xue.

llvm-svn: 356844