Add debug support symbols

[weliveindetail]

Proper ObjC support in debuggers requires the runtime to provide a few support symbols. Grepping the Apple V2 runtime in upstream LLDB brings up these candidates:

• gdb_object_getClass and gdb_objc_realized_classes for expression evaluation
  https://opensource.apple.com/source/objc4/objc4-787.1/runtime/objc-gdb.h.auto.html
• gdb_objc_trampolines and gdb_objc_trampolines_changed for trampoline step-through
  https://opensource.apple.com/source/objc4/objc4-437.1/runtime/objc-gdb.h.auto.html

I am not sure whether the referenced open-source declaration are the relevant sources or whether they are up-to-date.

[davidchisnall]

My understanding is that gdb_object_getClass is equivalent to object_getClass but doesn't acquire locks. Our implementation of this class never acquires locks, so we probably don't need it. I'm not sure what the others are for. I think the tampoline ones are specifically related to how Apple implements the objc_msgSend family, so I'd need to understand what the debugger wants from these to see what an equivalent would look like. gdb_objc_realized_classes depends on NXHashTable, which we don't implement (and I thought Apple removed with ObjC2).