gccrs: Add testcase for #[rustc_const_stable]

To ensure we don't introduce regressions back to issue #2314

gcc/testsuite/ChangeLog:

	* rust/compile/rustc_const_stable.rs: New test.

gcc/testsuite/rust/compile/rustc_const_stable.rs

@@ -0,0 +1,2 @@
+#[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
+pub fn foo() {} // { dg-error "macro not found" "" { target *-*-* } .-1 }