lldb: Report inferior signals as signals, not exceptions, on FreeBSD

This is the FreeBSD equivalent of LLVM 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

Submitted by: Karnajit Wangkhem
Obtained from: LLVM r310591