Add gdb script to print async stack trace

Summary:
This modifies the existing `co_bt` gdb script to make use of the new async stack traces available with folly::coro. This adds 2 new commands to gdb:

- `co_bt` with no arguments print the async stack trace for the current thread if there is an async operation in progress. This will print the normal stack frames interleaved with async stack frames as appropriate.
- `co_async_stack_roots` will print the `AsyncStackRoot` pointers available on the current thread
- `co_bt [ADDRESS]` will print the async stack frames starting at the provided `AsyncStackRoot` pointer

Reviewed By: andriigrynenko

Differential Revision: D26719625

fbshipit-source-id: 8edf6ab0851ab92614b3dbeafdf50a517dcc3a61