cmd/compile: eliminate direct uses of gc.Thearch in backends

cmd/compile: eliminate direct uses of gc.Thearch in backends

This CL changes the GOARCH.Init functions to take gc.Thearch as a
parameter, which gc.Main supplies.

Additionally, the x86 backend is refactored to decide within Init
whether to use the 387 or SSE2 instruction generators, rather than for
each individual SSA Value/Block.

Passes toolstash-check -all.

Change-Id: Ie6305a6cd6f6ab4e89ecbb3cbbaf5ffd57057a24
Reviewed-on: https://go-review.googlesource.com/38301
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>