tcg-mips: Move softmmu slow path out of line

At the same time, tidy up the call helpers, avoiding a memory reference.
Split out several subroutines.  Use TCGMemOp constants.  Make endianness
selectable at runtime.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>