rust: Drop libgcc dependency

Rustc unstable option "link-native-libraries" is used to prevent the
linkage of other libraries via the #[link] directive. Add a dependency
to libclang_rt.builtins, similarly to cc.

Bug: 141331117
Test: lunch aosp_crosshatch-userdebug; m
Change-Id: I5c232291a5dd08a99e6a12a1295e30bb8e4fcaf1
diff --git a/rust/config/toolchain.go b/rust/config/toolchain.go
index 9525c38..a769f12 100644
--- a/rust/config/toolchain.go
+++ b/rust/config/toolchain.go
@@ -112,6 +112,10 @@
 	return ""
 }
 
+func BuiltinsRuntimeLibrary(t Toolchain) string {
+	return LibclangRuntimeLibrary(t, "builtins")
+}
+
 func LibFuzzerRuntimeLibrary(t Toolchain) string {
 	return LibclangRuntimeLibrary(t, "fuzzer")
 }