Remove libgcc toolchain libs for Android

Remove Android-targeting gcc toolchain libraries -- libgcc,
libgcc_stripped, libatomic, and libgcov. Also remove libunwind_llvm,
which is replaced with a libunwind toolchain prebuilt.

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: I971d0a4a49f1aaeb3546e80b6d94208277a171ac