cmd/compile: tweak compiler concurrency

cmd/compile: tweak compiler concurrency

Default concurrency is now GOMAXPROCS, though this is normally
reduced to 4 by the go build commmand.

If compiler flags don't permit concurrency, silently
turn it down instead of failing the compilation.

Change-Id: Id26cc3214e0ca402d60f915e98276a58d809e097
Reviewed-on: https://go-review.googlesource.com/c/go/+/447358
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>