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
diff --git a/cc/cc_test.go b/cc/cc_test.go
index 76e75da..9fc1f35 100644
--- a/cc/cc_test.go
+++ b/cc/cc_test.go
@@ -2912,13 +2912,13 @@
 	// Check the shared version of lib2.
 	variant := "android_arm64_armv8-a_shared"
 	module := ctx.ModuleForTests("lib2", variant).Module().(*Module)
-	checkStaticLibs(t, []string{"lib1", "libc++demangle", "libclang_rt.builtins-aarch64-android", "libatomic"}, module)
+	checkStaticLibs(t, []string{"lib1", "libc++demangle", "libclang_rt.builtins-aarch64-android"}, module)
 
 	// Check the static version of lib2.
 	variant = "android_arm64_armv8-a_static"
 	module = ctx.ModuleForTests("lib2", variant).Module().(*Module)
 	// libc++_static is linked additionally.
-	checkStaticLibs(t, []string{"lib1", "libc++_static", "libc++demangle", "libclang_rt.builtins-aarch64-android", "libatomic"}, module)
+	checkStaticLibs(t, []string{"lib1", "libc++_static", "libc++demangle", "libclang_rt.builtins-aarch64-android"}, module)
 }
 
 var compilerFlagsTestCases = []struct {