Preparation for de-globalizing Stksize and MaxArg.

Passes toolstash -cmp. No compiler performance impact.

Updates #15756

Change-Id: I312f0bbd15587a6aebf472cd66c8e62b89e55c8a
Reviewed-on: https://go-review.googlesource.com/38328
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>