cmd/link: mangle function name with ABI on Mach-O

This is not strictly necessary. But as we already do this on
ELF and PE, do it here as well.

Updates #40724.

Change-Id: Ie6e5211aba116634bc9ed82eb8d22a7fed3b7776
Reviewed-on: https://go-review.googlesource.com/c/go/+/307229


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