[lldb] Add compile time checks for signal codes when on the matching platform

This adds a new macro to the UnixSignals subclasses, ADD_SIGCODE.

ADD_SIGCODE(4, ILL_ILLOPC, 1, "illegal opcode");

Adds a sigcode to signal 4. That code is ILL_ILLOPC and we expect
its value to be 1. When compiling on a system that matches the class
e.g. FreeBSD for FreeBSDSignals, the macro will check that that is true.

When you're not on FreeBSD we just use the number 1, and ILL_ILLOPC
won't be defined to anything because we don't include csignal.

Example error:
LinuxSignals.cpp:52:3: error: static_assert failed due to requirement
'ILL_COPROC == 9' "Value mismatch for signal code ILL_COPROC"

Reviewed By: arichardson

Differential Revision: https://reviews.llvm.org/D146222