cmd/link: update machoPlatform selection
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 <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

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