abuild: support arch="all !armhf"

this allows blacklisting architectures instead of just
whitelisting them. useful when more architectures comeabout.

abuild.in

@@ -1607,7 +1607,16 @@ srcpkg() {
 
 # return true if arch is supported or noarch
 check_arch() {
-	list_has $CARCH $arch || [ "$arch" = "noarch" ] || [ "$arch" = "all" ]
+	local ret=1
+	local i
+	for i in $arch; do
+		case $i in
+		all | noarch) ret=0 ;;
+		"$CARCH") ret=0 ;;
+		"!$CARCH") return 1 ;;
+		esac
+	done
+	return $ret
 }
 
 # return true if libc is not masked in options