Use the si_code value bionic passes us.

Bionic needs to re-raise various signals, which means the si_code
debuggerd sees has been clobbered. If bionic sends us the original
si_code value, we can use that instead of the one we see when the
ptrace the crashed process' siginfo.

Change-Id: If116a6bc667d55a6fb39b74f96673292af4e4c8c