Libjit x86 back end enabled

Hi all,

The CVS version of libjit now has the x86 back end enabled by default.  All of 
the tutorials and existing test cases work with both the interpreter and x86.  
If you run "./configure" and "make" on an x86 machine, you will get  real 
JIT!  You can use "./configure --enable-interpreter" to force the interpreted 
version of libjit, for comparison purposes.

There are still a number of instructions that need expansion for x86 to bring 
it in line with the interpreter's capabilities, but it is very close.  And 
the ARM back end is about half done.

The level of optimization is fairly modest at the moment: simple local 
register allocation and variable dependency analysis.  But it does generate 
pretty good x86 code despite that, and should beat the CVM unroller quite 
comfortably when it is integrated into pnet.

Once the interpreter, x86, and ARM back ends are working well at this level of 
optimization, then I'll be looking into other optimizations ("make it work 
and then make it work better", as always).

Please let me know if there are any problems.

Cheers,

Rhys.