rulegen has a sanity check that ensures all the arch-specific opcodes
are handled by each of the gen files.

This is an expensive chunk of work, particularly since there are a lot
of opcodes in total, and each one of them compiles and runs a regular
expression.

Parallelize that for each architecture, which greatly speeds up 'go run
*.go' on my laptop with four real CPU cores.

	name     old time/op         new time/op         delta
	Rulegen          3.39s ± 1%          2.53s ± 2%  -25.34%  (p=0.008 n=5+5)

	name     old user-time/op    new user-time/op    delta
	Rulegen          10.6s ± 1%          11.2s ± 1%   +6.09%  (p=0.008 n=5+5)

	name     old sys-time/op     new sys-time/op     delta
	Rulegen          201ms ± 7%          218ms ±17%     ~     (p=0.548 n=5+5)

	name     old peak-RSS-bytes  new peak-RSS-bytes  delta
	Rulegen          182MB ± 3%          184MB ± 3%     ~     (p=0.690 n=5+5)

Change-Id: Iec538ed0fa7eb867eeeeaab3da1e2615ce32cbb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/195218
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>