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

bp2build/cc_library_headers_conversion_test.go

@@ -37,17 +37,6 @@ toolchain_library {
 	recovery_available: true,
 	native_bridge_supported: true,
 	src: "",
-}
-
-toolchain_library {
-	name: "libatomic",
-	defaults: ["linux_bionic_supported"],
-	vendor_available: true,
-	vendor_ramdisk_available: true,
-	product_available: true,
-	recovery_available: true,
-	native_bridge_supported: true,
-	src: "",
 }`
 )