internal/cpu: correctly link to getsystemcfg

Directly set getsystemcfg as //extern in internal/cpu instead of
trying to use the runtime as in Go toolchain.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/287932

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-2663206528a6d46cbde60dbccf84c8288707ab8d
+271a043537f2f0ae93bde2cf4f4897e68a476ece
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/internal/cpu/cpu_ppc64x_aix.go

@@ -17,5 +17,5 @@ func osinit() {
 	PPC64.IsPOWER9 = isSet(impl, _IMPL_POWER9)
 }
 
-// getsystemcfg is defined in runtime/os2_aix.go
+//extern getsystemcfg
 func getsystemcfg(label uint) uint

libgo/go/runtime/os_aix.go

@@ -13,9 +13,6 @@ import (
 //extern sysconf
 func sysconf(int32) _C_long
 
-//extern getsystemcfg
-func getsystemcfg(int32) uint64
-
 type mOS struct {
 	waitsema uintptr // semaphore for parking on locks
 }