target-arm: fix sdiv helper

(INT32_MIN / -1) triggers an overflow, and the result depends on the
host architecture (INT32_MIN on arm, -1 on ppc, SIGFPE on x86). Use a
test to output the correct value.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Acked-by: Laurent Desnogues <laurent.desnogues@gmail.com>
