[CVS ci] JIT support for JITed functions that call external functions

[Leopold Toetsch]

Thanks to Jeff, who pointed at that, Parrot's JIT system now explicitely 
supports JITed functions that call into Parrot, like e.g. most string 
functions do.

Here are the steps to use this functionality:
1) Arrange mappable registers like this:
   char <type>val_map[] = { non-volatiles, volatiles }:
2) define PRESERVED_<uc_type>_REGS as the count of non-volatiles
3) denote a function call in core.jit as
   CALL_FUNCTION(jit_info, the_function);
NB: the only effect of this statement is that the extcall flag of that 
opcode is set to -1. It does not call the function or even try to setup 
function arguments. Function call is done by ...

4) define that macro on top of core.jit
jit/i386/core.jit is using this functionality now.
Have fun,
leo