s390x doesn't introduce any new assembly syntax. There are a few
instructions which require the operands to be reordered, notably
the storage-storage instructions that put the length into From3 so
that the memory operands can be put into From and To.

The assembly test currently covers a subset of instructions but
tries to hit edge cases as much as possible. Unlike the other ports
it can be linked as an executable to make disassembling it easy.
It would be nice to autogenerate it at some point in the future.

Change-Id: I7615ac6ecf239e3f347fad9ae1f8eede91742859
Reviewed-on: https://go-review.googlesource.com/20934
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>