Turn stable-features error into a warning

This CL turns the stable-features lint into a warning.  This is
necessary to complete toolchain updates until we finish enforcing the
"no unstable features policy".

Test: ./test_compiler --prebuilt-path ../../dist/rust-1.66.1.tar.gz
Bug: 267698452
Bug: 263153841
Change-Id: Ib095dea045111ad3d89327a694aa5fa5b7f3128c

rust/config/global.go

@@ -51,6 +51,9 @@ var (
 		// Use v0 mangling to distinguish from C++ symbols
 		"-C symbol-mangling-version=v0",
 		"--color always",
+		// TODO (b/267698452): Temporary workaround until the "no unstable
+		// features" policy is enforced.
+		"-A stable-features",
 	}
 
 	deviceGlobalRustFlags = []string{