cmd/compile: move stringtoslicebytetmp to the backend

cmd/compile: move stringtoslicebytetmp to the backend

- removes the runtime function stringtoslicebytetmp
- removes the generation of calls to stringtoslicebytetmp from the frontend
- adds handling of OSTRARRAYBYTETMP in the backend

This reduces binary sizes and avoids function call overhead.

Change-Id: Ib9988d48549cee663b685b4897a483f94727b940
Reviewed-on: https://go-review.googlesource.com/32158
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>