[dev.link] cmd/link run generators in parallel

[dev.link] cmd/link run generators in parallel

Small runtime win:

Stats for darwin, building cmd/compile:

Asmb                      20.7ms ±14%    18.3ms ±14%  -11.54%  (p=0.002 n=10+10)
TotalTime                  365ms ±10%     351ms ± 2%     ~     (p=0.211 n=10+9)

Change-Id: Ia8afdf6948111d59b0c89e52cb50557a10f33c40
Reviewed-on: https://go-review.googlesource.com/c/go/+/257964
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>