target-s390x: fix CONVERT TO BINARY (CVD, CVDY)

current_number being shift left by more than 32 bits, we can't use a
simple int. Similarly use an int64_t type for the input binary value,
to not get the -2^31 case wrong. Finally don't initialize shift to 4,
it's already done in the for loop.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
