cmd/link: use internal linking for -race mode on darwin/arm64

The code I wrote in ldmacho.go in CL 266373 was plainly wrong. It
didn't carry rAdd over correctly. Fixed. Also added sign extension
(as ld64 does).

Internal linking with -race mode now works. Enable it.

Updates #38485.

Change-Id: I78aa949687bf6a0987913059059160b018c7560e
Reviewed-on: https://go-review.googlesource.com/c/go/+/267097


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