[RFA] Patch to sim/common/cgen.sh

[Ben Elliston]

   This patch adds a new "defs" action for generating a "defs" file
   containing per-ISA definitions needed by multi-ISA simulators.  In
   addition, I've made a couple of minor improvements -- the largest
   being the factorisation of sed expressions into a single (more
   maintainable) shell variable.  This is a re-working of this patch,
   since Doug Evans pointed out that my last re-write was not safe for
   concurrent runs.

Approved by Doug Evans, so I'm going to check this in.

Ben