Chain the temporaries together via pointers intstead of indices.
The mem_reg value is now mem_base->reg. This will be important later.
This does require that the frame pointer have a global temporary
allocated for it. This is simple bar the existing reserved_regs check.
Backports commit b3a62939561e07bc34493444fa926b6137cba4e8 from qemu
Thus, use cpu_env as the parameter, not TCG_AREG0 directly.
Update all uses in the translators.
Backports commit e1ccc05444676b92c63708096e36582be27fbee1 from qemu