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

[DJ Delorie]

> Any reason why I can't/shouldn't use CC_FOR_BUILD ??

Because it's not defined by configure.  Look for it in your build's
opcodes/Makefile and you'll see this:

	CC_FOR_BUILD = @CC_FOR_BUILD@

The only time you can use this is when you call "make" from the top of
the tree, and only because the top Makefile overrides this when it
runs opcodes's Makefile.  However, if you type "make" in opcodes, it
will break.

If you want to add logic to set CC_FOR_BUILD properly in
opcodes/configure, that would be OK.