When a cgo program calls setuid, setgid, etc., the GNU/Linux
pthread library sends signal SIGSETXID to each thread to tell
it to update its UID info.  If Go is permitted to intercept
the default SIGSETXID signal handler, the program will hang.

This patch tells the runtime package to not try to intercept
SIGSETXID on GNU/Linux.  This will be odd if a Go program
wants to try to use that signal, but it means that cgo
programs that call setuid, etc., won't hang.

Fixes #3871.

R=rsc, r, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/6455050