Remove clang: false support

Most everything in Soong is using clang (and we're getting very close
for Make as well), so remove the ability to select GCC to build a
module.

We still need to keep the internal functionality around to find libgcc.a
and other toolchain_library modules with GCC.

Test: m blueprint_tools
Change-Id: I5ef3e3836b9ad3e160669ac32aee39698c9b72c3

cc/toolchain_library.go

@@ -43,7 +43,7 @@ func toolchainLibraryFactory() android.Module {
 	}
 	module.compiler = toolchainLibrary
 	module.linker = toolchainLibrary
-	module.Properties.Clang = BoolPtr(false)
+	module.Properties.Gcc = true
 	module.stl = nil
 	module.sanitize = nil
 	module.installer = nil