cpu: add missing linkname for libc_getsystemcfg on aix/ppc64

This fixes the aix/ppc64 build after CL 179178

Change-Id: Ie8d762b533c05d6ca262030b34ae137140896d44
Reviewed-on: https://go-review.googlesource.com/c/sys/+/179317
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

cpu/syscall_aix_ppc64_gc.go

@@ -18,6 +18,8 @@ import (
 
 //go:cgo_import_dynamic libc_getsystemcfg getsystemcfg "libc.a/shr_64.o"
 
+//go:linkname libc_getsystemcfg libc_getsystemcfg
+
 type syscallFunc uintptr
 
 var libc_getsystemcfg syscallFunc