syscall: use Getpgid to emulate Getpgrp on linux

syscall: use Getpgid to emulate Getpgrp on linux

Fixes newly introduced test on linux/arm64 because linux/arm64 doesn't
have the getpgrp syscall. Getpgid(0) is documented to be equivalent to
Getpgrp.

Change-Id: I8f30f4f8de8c32fe04a29c9c4a9330d4e4e6b46d
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8022
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Rob Pike <r@golang.org>