syscall: fix TestForeground for AIX

syscall: fix TestForeground for AIX

On AIX, sys.Pgid must be a int32 and not a int64 as on Solaris for ioctl
syscall.
Pid_t type can be used to provide the same code in both OS. But pid_t
must be added to ztypes_solaris_amd64.go.

Change-Id: I1dbe57f099f9e5ac9491aaf246a521137eea5014
Reviewed-on: https://go-review.googlesource.com/c/144539
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>