unix: add Getsystemcfg on aix

This will be used to detect POWER8/POWER9 support in x/sys/cpu

Updates golang/go#30732

Change-Id: I10ad6e440920c34c378bf6783417bc09421ff500
Reviewed-on: https://go-review.googlesource.com/c/sys/+/166465
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_aix.go

@@ -545,3 +545,5 @@ func Poll(fds []PollFd, timeout int) (n int, err error) {
 //sys	gettimeofday(tv *Timeval, tzp *Timezone) (err error)
 //sysnb	Time(t *Time_t) (tt Time_t, err error)
 //sys	Utime(path string, buf *Utimbuf) (err error)
+
+//sys	Getsystemcfg(label int) (n uint64)