tcg/arm: Fix double-word comparisons

The code sequence we were generating was only good for unsigned
comparisons.  For signed comparisions, use the sequence from gcc.

Fixes booting of ppc64 firmware, with a patch changing the code
sequence for ppc comparisons.

Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>