Generating CPU family headers for multiple isas

[Ben Elliston]

fche wrote:

   (The sid application generates only isa-independent goods into cpu.h,
   so this problem does not appear there.  sfmts, insn enums, parexec
   etc. all go into isa-specific defs.h files.)

At first I thought this sounded ridiculously complicated and difficult.
After much tinkering today, I came to the conlusion that this was the right
way to proceed for the sim framework, and I've managed to do it!  Patches
will be following tomorrow once I've had a chance to test and document the
changes.

For what it's worth, by the way, the sim/common/cgen.sh script really needs
a major overhaul.  It's a mess.

Ben