mips/aaccoder: use variables instead of using register names directly

On mips64, the registers t[4-7] do not exist. Instead of using a lot of #ifdef
or defines to handle differing register names, use variables and let GCC
allocate the registers automatically (like in the other mips assembly files).

In get_band_cost_ESC_mips, t4 and t5 were renamed to t6 and t7 to avoid a
variable name conflict.
Signed-off-by: James Cowgill <james410@cowgill.org.uk>
Reviewed-by: Nedeljko Babic <Nedeljko.Babic@imgtec.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>