Add -latomic to all target builds

Projects using stdatomic.h needs libatomic.a in case compiler can't
expand all __atomic* intrinsics. eg, __atomic_is_lock_free in
armeabi/mips.

Adding libatomic.a globally makes more sense than adding
"LOCAL_LDLIB += -latomic " in each project including <stdatomic.h>.
Projects don't need atomic operations won't get redundant DT_NEEDED
entry because libatomic.a is not a shared library.

Change-Id: I81dbf524544c848e667e18ab5eeabff75b5063ef