tcg: Change ts->mem_reg to ts->mem_base

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.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
