[Lightning] Allow use of function argument registers

[Paul Cercueil]

Hi Paulo,

On a lot of archs there are registers dedicated to passing arguments to 
functions. These are obviously not part of the 'R' or 'V' pools.

However, it would be great to still be able to use these registers, as 
part of a 'A' pool maybe, as long as it's understood that they get 
trashed at each call to jit_pusharg() (and are caller-saved).

On MIPS with the n32 ABI, that's 8 registers that aren't usable...

Thoughts?

-Paul