runtime,runtime/pprof: get memory mappings on darwin.

runtime,runtime/pprof: get memory mappings on darwin.

Displaying assembly language has never worked for Apple Silicon
macs (see #50891). This change uses mach_vm_region to obtain the
necessary VM mappings to allow for locating assembly instructions
for a cpu profile.

Fixes #50891

Change-Id: Ib968c55a19b481b82f63337276b552f3b18f69d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/503919
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>