Store thunk regs on the stack, not a global.

This may even be more near active memory anyway, but it reduces the "64
bit address out of range" RIP issues a bit too.  It's currently impossible
to enable appverif memory checks and jit in 64-bit...

Avoided the 32-bit path because I'm less sure about alignment.
Merge pull request #5453 from unknownbrackets/jit-minor

Store thunk regs on the stack, not a global