PtracePeekUser was already added in CL 38638, also add its "counterpart"
allowing to modify the tracee's user area.

Change-Id: Id84e85fe17b5cab7df5ecc83b6b7b8a39241241d
Reviewed-on: https://go-review.googlesource.com/76670Reviewed-by: Ian Lance Taylor <iant@golang.org>