cmd/internal/obj: eagerly initialize assemblers

cmd/internal/obj: eagerly initialize assemblers

CL 38662 changed the x86 assembler to be eagerly
initialized, for a concurrent backend.

This CL puts in place a proper mechanism for doing so,
and switches all architectures to use it.

Passes toolstash-check -all.

Updates #15756

Change-Id: Id2aa527d3a8259c95797d63a2f0d1123e3ca2a1c
Reviewed-on: https://go-review.googlesource.com/39917
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>