gcc: make gnu-unique-object handling explicit

this is generally auto-enabled with glibc >2.11, but declare
intent for more reliable crossbuilds

srcpkgs/gcc/template

@@ -235,9 +235,12 @@ do_configure() {
 	fi
 
 	if [ "$XBPS_TARGET_LIBC" = "musl" ]; then
+		_args+=" --disable-gnu-unique-object"
 		_args+=" --disable-libsanitizer"
 		_args+=" --disable-symvers"
 		_args+=" libat_cv_have_ifunc=no"
+	else
+		-args+=" --enable-gnu-unique-object"
 	fi
 
 	case "$XBPS_TARGET_MACHINE" in