[dev.regabi] cmd/go: Use GOMAXPROCS to limit default build, compile parallelism

When people want deterministic/single-process builds, they probably
assume that GOMAXPROCS=1 will do that.  It currently does not,
neither for build parallelism nor for compiler internal parallelism.
(Current incantation for that is "go build -p=1 -gcflags=all=-c=1 ... ")

This CL makes
  "GOMAXPROCS=1 go build ..."
behave like
  "go build -p=1 -gcflags=all=-c=1 ... "

RELNOTE=yes

Change-Id: I9cfe50b7deee7334d2f1057b58385f6c98547b9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/284695


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>