userland: add assembly support

Move arm assembly cheat here, and start some work on x86 cheat as well.

build-userland-in-tree

@@ -1,2 +1,7 @@
 #!/usr/bin/env bash
-"$(git rev-parse --show-toplevel)/build-userland" --gcc-which host --in-tree "$@"
+"$(git rev-parse --show-toplevel)/build-userland" \
+  --gcc-which host \
+  --in-tree \
+  --target-cwd \
+  "$@" \
+;