Some Linux architectures (e.g. arm64) don't have the getpgrp syscall,
use getpgid(0) to emulate it.

Update golang/go#10150.

This brings CL 8022 that has been applied to the syscall package to
x/sys.

Change-Id: I24c6d7e8b5b2f075ca4e68b142b2e03ab8a43342
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8023Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>