[dev.link] cmd/link: allow OutBufs to work outside mmapped area

[dev.link] cmd/link: allow OutBufs to work outside mmapped area

Asmb                      9.76ms ±13%    9.91ms ±16%     ~     (p=0.912 n=10+10)
Munmap                    16.0ms ± 8%    18.0ms ±53%     ~     (p=0.203 n=8+10)
Asmb2                     2.30ms ± 6%    2.21ms ±14%     ~     (p=0.095 n=10+9)

Future changes will add fallocate on supported platforms, and eliminate
Msync.

Change-Id: I6fc35fb2739c8530c8732c3ad13c99e6004de04a
Reviewed-on: https://go-review.googlesource.com/c/go/+/228197
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>