After r364753, there should be no need to suppress -Watomic-alignment
warnings anymore for compiler-rt's atomic.c. This occurred because the
IS_LOCK_FREE_8 macro was not correctly defined to 0 for mips, and this
caused the compiler to emit a runtime call to __atomic_is_lock_free(),
and that triggers the warning.

MFC after: 2 weeks
X-MFC-With: r364753

(cherry picked from commit 0709bb1f4146361e7917201a2438e1b2cd36f40b)