tcg: initial mips support

Based on a patch from Arnaud Patard (Rtp) <arnaud.patard@rtp-net.org>

A few words about design choices:
* Two registers, at and t0, are reserved for TCG internal use. They are
  useful for bswap and 64-bit ops.
* Most ops supports a constant argument with value 0, which is actually
  mapped to the zero register.
* While the at register is available for constant loading, ops only
  support a limited range of constants. TCG does a better job doing the
  register allocation and constant loading by itself. There are plenty of
  registers available anyway.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>