Stop linking against libatomic

The __atomic_* library functions are now part of the compiler-rt
builtins library:

https://android-review.googlesource.com/c/toolchain/llvm_android/+/1625025

Bug: http://b/153025717
Test: treehugger
Change-Id: Iebd5eaf17f8559696b6d6e2e0dd782f77b6bcab5