mesa: actually enable _have_arm

this accidentally got missed during a refactor, sorry

srcpkgs/mesa/template

@@ -1,7 +1,7 @@
 # Template file for 'mesa'
 pkgname=mesa
 version=20.0.5
-revision=3
+revision=4
 wrksrc="mesa-${version}"
 build_style=meson
 configure_args="-Dglvnd=true -Dshared-glapi=true -Dgbm=true -Degl=true
@@ -70,6 +70,9 @@ case "$XBPS_TARGET_MACHINE" in
 		_have_vmware=yes
 		_have_nine=yes
 		;;
+	armv[67]*|aarch64*)
+		_have_arm=yes
+		;;
 esac
 
 if [ "$_have_amd" ]; then