cmd/link: update machoPlatform selection

Now that we have darwin/arm64 and ios/arm64 ports, make it based
on GOOS, instead of GOARCH.

Also drop a remaining case of 32-bit ARM.

Change-Id: I954fff980712fd3b81b561ddcb6f3a4ef73fa0b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/312549


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