The return is by reference, not in 4 integer registers.

This error resulted in

  qemu-system-i386: tcg/mips/tcg-target.c.inc:140: \
    tcg_target_call_oarg_reg: Assertion `slot >= 0 && slot <= 1' failed.

Fixes: 5427a9a7 ("tcg: Add TCG_TARGET_CALL_{RET,ARG}_I128")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>