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

apex/apex.go

@@ -2925,9 +2925,7 @@ func makeApexAvailableBaseline() map[string][]string {
 		"com.google.android.material_material",
 		"com.google.android.material_material-nodeps",
 
-		"libatomic",
 		"libclang_rt",
-		"libgcc_stripped",
 		"libprofile-clang-extras",
 		"libprofile-clang-extras_ndk",
 		"libprofile-extras",