The Platform base class now maintains a list of trap handlers
aka asynchronous signal handlers, which subclasses should fill
in as appropriate.  For most Unix user process environments,
the one entry in this list is _sigtramp.  For bare-board and
kernel environments, there will be different sets of trap
handlers.

The unwinder needs to know when a frame is a trap handler
because the rules it enforces for the frame "above" the
trap handler is different from most middle-of-the-stack frames.

<rdar://problem/15835846>