cmd/internal/obj: eagerly initialize x86 assembler

cmd/internal/obj: eagerly initialize x86 assembler

Prior to this CL, instinit was called as needed.
This does not work well in a concurrent backend.
Initialization is very cheap; do it on startup instead.

Passes toolstash-check -all.
No compiler performance impact.

Updates #15756

Change-Id: Ifa5e82e8abf4504435e1b28766f5703a0555f42d
Reviewed-on: https://go-review.googlesource.com/38662
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>