[dev.link] cmd/link: parallelize dwarfcompress

[dev.link] cmd/link: parallelize dwarfcompress

Benchmarked on cmd/compile (n=10) on my laptop. Total time is a 15%
improvement:

name                    old time/op    new time/op     delta
Dwarfcompress              181ms ± 7%       62ms ± 7%       -66.01%  (p=0.000 n=10+10)
TotalTime                  865ms ± 5%      742ms ± 4%       -14.19%  (p=0.000 n=10+10)

name                    old alloc/op   new alloc/op    delta
Dwarfcompress             24.5MB ± 0%     24.5MB ± 0%        +0.01%  (p=0.000 n=10+10)

name                    old allocs/op  new allocs/op   delta
Dwarfcompress                344 ± 0%        350 ± 1%        +1.72%  (p=0.000 n=10+10)

name                    old heap-B     new heap-B      delta
Dwarfcompress               164M ± 0%       164M ± 0%          ~     (p=0.232 n=8+7)

Change-Id: Id42033073b021d6a4e1fc75a2f6b7f3605fe5d2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/220843
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>