[dev.link] cmd/link: emit Mach-O relocations in mmap

[dev.link] cmd/link: emit Mach-O relocations in mmap

Following CL 240399 and CL 240400, do the same for Mach-O.

Linking cmd/compile with external linking,

name         old time/op    new time/op    delta
Asmb2_GC       32.7ms ± 2%    13.5ms ± 6%   -58.56%  (p=0.008 n=5+5)

name         old alloc/op   new alloc/op   delta
Asmb2_GC       16.5MB ± 0%     6.4MB ± 0%   -61.15%  (p=0.008 n=5+5)

Change-Id: I0fd7019d8713d1940e5fbbce4ee8eebd926451a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/241178
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>