[dev.link] cmd/link: fallocate space, and remove all msync calls

[dev.link] cmd/link: fallocate space, and remove all msync calls

The fallocate calls will lower the chances of SIGBUS in the linker, but
it might still happen on other unsupported platforms and filesystems.

Darwin cmd/compile stats:

Munmap                    16.0ms ± 8%     0.8ms ± 3%   -95.19%  (p=0.000 n=8+10)
TotalTime                  484ms ± 2%     462ms ± 2%    -4.52%  (p=0.000 n=10+9)

Updates #37310

Change-Id: I41c6e490adec26fa1ebee49a5b268828f5ba05e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/228385
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>