[m-dev.] bug fix to frameopt

[Zoltan Somogyi]

> What about pragma_c's that call back Mercury code?
> Will they also clobber the succip?

Good question.  I though they couldn't, but it seems I was wrong.
I will fix frameopt accordingly.

David and I just looked at the template in pragma_c_gen, and we
couldn't quite figure out why for may_call_mercury pragma C code,
the restore_registers was conditional on #ifndef CONSERVATIVE_GC
but the corresponding save_registers wasn't. We thought it was
to prepare for the restore_registers that is done when calling
Mercury from C, but we weren't sure. Whether we were correct or not,
the reason should be documented in pragma_c_gen.m.

Zoltan.