[dev.link] cmd/link: make asmb parallel across all platforms

[dev.link] cmd/link: make asmb parallel across all platforms

Numbers for x86:

Asmb                      54.8ms ±15%    12.1ms ±28%   -78.02%  (p=0.000 n=10+10)
TotalTime                  545ms ± 3%     515ms ± 4%    -5.42%  (p=0.000 n=10+10)

Change-Id: I8fc17f7ccdf83fbd70cf7d27b44f41f9db9c9922
Reviewed-on: https://go-review.googlesource.com/c/go/+/224419
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>