[dev.ssa] cmd/compile: port SSA backend to amd64p32

[dev.ssa] cmd/compile: port SSA backend to amd64p32

It's not a new backend, just a PtrSize==4 modification
of the existing AMD64 backend.

Change-Id: Icc63521a5cf4ebb379f7430ef3f070894c09afda
Reviewed-on: https://go-review.googlesource.com/25586
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>