[asan] fix the reported PCs for powerpc64

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182477 91177308-0d34-0410-b5e6-96231b3b80d8
Effectively revert r182453 - atomic.c may not be compiled if host compiler doesn't understand _Atomic

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182478 91177308-0d34-0410-b5e6-96231b3b80d8