cmd/compile: fix wrong package path for unsafe.Pointer

It's not a predeclared type, but a type defined in "unsafe" package.

Fixes #44830

Change-Id: If39815b1070059b608be8231dfac9b7f3307cb15
Reviewed-on: https://go-review.googlesource.com/c/go/+/313349


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>