Adds Prctl syscall for linux platforms. syscall and x/sys/unix already
defined the PR_ series of constants.

See discussion: https://groups.google.com/forum/#!topic/golang-nuts/rG6xShds2Dc

Change-Id: Ida5562b46abef574a3f9ac31911142f1e8d7fb4f
Reviewed-on: https://go-review.googlesource.com/15520Reviewed-by: Ian Lance Taylor <iant@golang.org>