cmd/link: enable internal linking by default on darwin/arm64

With previous CLs, internal linking without cgo should work well.
Enable it by default. And stop always requiring cgo.

Enable tests that were previously disabled due to the lack of
internal linking.

Updates #38485.

Change-Id: I45125b9c263fd21d6847aa6b14ecaea3a2989b29
Reviewed-on: https://go-review.googlesource.com/c/go/+/265121


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>