gcc: patch out __gnu_linux__ for ppc*-musl

This macro should not be defined on non-glibc systems. It's not
defined on non-glibc systems on other architectures and has been
removed upstream, too.

[ci skip]

srcpkgs/gcc/template

@@ -163,6 +163,12 @@ pre_configure() {
 	case "$XBPS_TARGET_MACHINE" in
 		*-musl) patch -p1 -i ${FILESDIR}/libgnarl-musl.patch ;;
 	esac
+	# REMOVE WITH 9.1
+	case "$XBPS_TARGET_MACHINE" in
+		ppc*-musl)
+			sed -i 's/ \-D__gnu_linux__//' gcc/config/rs6000/sysv4.h
+			;;
+	esac
 }
 do_configure() {
 	local _langs _args _hash