This makes the script resistant to changes in the SDK location, which
apparently happens from time to time.

While I was at it, made another cosmetic touch-up: use a .gitignore file
to stop `git status` from showing the _obj/ directory generated by
mkall.sh.

Change-Id: Ia706114d94e324ab443f6f5d6aa7c0c9f8a84620
Reviewed-on: https://go-review.googlesource.com/14192Reviewed-by: Ian Lance Taylor <iant@golang.org>