tcp/mips: Change TCG_AREG0 (fp -> s0)

Register fp (frame pointer) is a bad choice for compilations
without optimisation, because the compiler makes heavy use
of this register (so the resulting code crashes).

Register s0 had been used for TCG_AREG1 in earlier releases,
but was no longer used and is now free for TCG_AREG0.

The resulting code works for compilations without
optimisation (tested with qemu mips in qemu mips
on x86 host).

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
