Decode ptrace-induced SIGTRAP si_code values.

Example:

  signal 5 (SIGTRAP), code -32763 (PTRACE_EVENT_STOP), fault addr 0x274e00005fb3

I'm tempted to say that %d isn't the best choice for si_code, but as long as
we're fully decoding all the values, I don't think it matters.

Bug: http://b/62856172
Test: manual debuggerd run
Change-Id: Ieeca690828e1e12f4162bbadece53f4aa7b9537a