unix: add PtracePokeUser
unix: add PtracePokeUser

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/76670
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys