gccrs: attributes: Add #[macro_use] as builtin

gcc/rust/ChangeLog:

	* util/rust-attributes.cc: Add `macro_use` to list of builtin
	attributes.

gcc/testsuite/ChangeLog:

	* rust/compile/macro_export_1.rs: New test.

gcc/rust/util/rust-attributes.cc

@@ -40,6 +40,7 @@ static const BuiltinAttrDefinition __definitions[]
      {"no_mangle", CODE_GENERATION},
      {"repr", CODE_GENERATION},
      {"path", EXPANSION},
+     {"macro_use", NAME_RESOLUTION},
      // From now on, these are reserved by the compiler and gated through
      // #![feature(rustc_attrs)]
      {"rustc_inherit_overflow_checks", CODE_GENERATION}};

gcc/testsuite/rust/compile/macro_export_1.rs

@@ -0,0 +1,2 @@
+#[macro_use]
+mod foo {}