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

cc/linker.go

@@ -321,10 +321,9 @@ func (linker *baseLinker) linkerDeps(ctx DepsContext, deps Deps) Deps {
 	}
 
 	if ctx.toolchain().Bionic() {
-		// libclang_rt.builtins and libatomic have to be last on the command line
+		// libclang_rt.builtins has to be last on the command line
 		if !Bool(linker.Properties.No_libcrt) {
 			deps.LateStaticLibs = append(deps.LateStaticLibs, config.BuiltinsRuntimeLibrary(ctx.toolchain()))
-			deps.LateStaticLibs = append(deps.LateStaticLibs, "libatomic")
 		}
 
 		deps.SystemSharedLibs = linker.Properties.System_shared_libs