rust: kbuild: support skipping flags in rustc_test_library

Crates like `quote` (added later) will need the ability to skip flags
in the `rustc_test_library` command.

Thus add the support for it.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/Makefile

@@ -172,8 +172,8 @@ rustdoc-clean: FORCE
 quiet_cmd_rustc_test_library = $(RUSTC_OR_CLIPPY_QUIET) TL $<
       cmd_rustc_test_library = \
 	OBJTREE=$(abspath $(objtree)) \
-	$(RUSTC_OR_CLIPPY) $(rust_common_flags) \
-		@$(objtree)/include/generated/rustc_cfg $(rustc_target_flags) \
+	$(RUSTC_OR_CLIPPY) $(filter-out $(skip_flags),$(rust_common_flags) $(rustc_target_flags)) \
+		@$(objtree)/include/generated/rustc_cfg \
 		--crate-type $(if $(rustc_test_library_proc),proc-macro,rlib) \
 		--out-dir $(objtree)/$(obj)/test --cfg testlib \
 		-L$(objtree)/$(obj)/test \