Automatic opcode table generation and gas test cases for s/390.

[DJ Delorie]

> Hmm, if --host doesn't match --build it doesn't work though. Too bad. Any
> idea how to fix this?

1. Use a portable shell script (ick).

2. Go through gyrations to find a native compiler (like gcc does).

3. Make it a maintainer-mode-only thing, and also stipulate native
   only in configure, so it just doesn't run when it can't (you'd have
   to worry about updating a file in the source tree, which might be
   on a read-only filesystem).

4. Just don't do that :-)

I'm not sure which makes the most sense, though.