runtime: fix SI_USER for FreeBSD Ref: http://svnweb.freebsd.org/base/head/sys/sys/signal.h?revision=HEAD&view=markup
runtime: fix SI_USER for FreeBSD
Ref: http://svnweb.freebsd.org/base/head/sys/sys/signal.h?revision=HEAD&view=markup

R=golang-dev, devon.odell, r
CC=golang-dev
http://codereview.appspot.com/6490118
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go