[Lightning] jit_allocai is there

[Paolo Bonzini]

Okay, thanks to Ludovic who started the effort, we have a way to declare 
stack space in lightning.

Documentation will come later.

There is a big caveat against using jit_allocai and jit_pushr/popr 
together: there is no guarantee that JIT_FP is not the stack pointer.  
Therefore, on the PPC for example, between a jit_pushr and a jit_popr, 
the offsets returned by jit_allocai become invalid.  It might be the 
same on the i386; since we were already using EBP to access the 
arguments I took the easier path, but if we could really remove 
jit_pushr/popr we could access arguments and stack variables with ESP.

Paolo