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.

Change-Id: I2a095fd471f20be33205c160607b749dd04a017c
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/287932
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Zhang <cherryyz@google.com>