Print si_code even if there's no string decoding available.
Print si_code even if there's no string decoding available.

Summary: We are getting SIGILLs that end up not printing a code because it's not in the cases in `signal_reason`, which isn't particularly helpful. Additionally, the check for kernel-sourced si_codes is incorrect. Any code > 0 is from the kernel. This causes folly to log nonsense PIDs and UIDs.

Differential Revision: D14388102

fbshipit-source-id: 3201a16bbe9eb9038ebc5601044406f5333ec9e9

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly