gdb: handle missing struct pthread definition when finding async stack traces
gdb: handle missing struct pthread definition when finding async stack traces

Summary:
Depending on the environment, the type definition for `struct pthread`
(pthread implementation detail for `pthread_t`) may not be available in gdb.
If this happens, use a predefined offset for linux x86_64 to find the `specific`
field inside `struct pthread` that holds the thread-local storage values.

Reviewed By: yfeldblum

Differential Revision: D27112799

fbshipit-source-id: 38c88210c3cd2a6b06918de33167d862f59a676e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly