cmd/compile: builtins may be in the unsafe package

cmd/compile: builtins may be in the unsafe package

Now that unsafe.Sizeof and friends can operate on generic parameters,
and evaluate to non-constants, we need to export/import them correctly.

Fixes #48094

Change-Id: If3ebf77255385cd5462e13fb7ced8b157ba3cf5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/346469
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>