Report inferior signals as signals, not exceptions, on FreeBSD

This is the FreeBSD equivalent of r238549.

This serves 2 purposes:

* LLDB should handle inferior process signals SIGSEGV/SIGILL/SIGBUS/
  SIGFPE the way it is suppose to be handled. Prior to this fix these
  signals will neither create a coredump, nor exit from the debugger
  or work for signal handling scenario.
* eInvalidCrashReason need not report "unknown crash reason" if we have
  a valid si_signo

llvm.org/pr23699

Patch by Karnajit Wangkhem

Differential Revision:	https://reviews.llvm.org/D35223

llvm-svn: 310591