cmd/compile: make non-concurrent compiles deterministic again

Spreading function compilation across multiple goroutines results in
non-deterministic output. This is how cmd/compile has historically
behaved for concurrent builds, but is troublesome for non-concurrent
builds, particularly because it interferes with "toolstash -cmp".

I spent some time trying to think of a simple, unified algorithm that
can concurrently schedule work but gracefully degrades to a
deterministic build for single-worker builds, but I couldn't come up
with any. The simplest idea I found was to simply abstract away the
operation of scheduling work so that we can have alternative
deterministic vs concurrent modes.

Change-Id: I08afa00527ce1844432412f4f8553781c4e323df
Reviewed-on: https://go-review.googlesource.com/c/go/+/318229
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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