Issue #12605: Show information on more C frames within gdb backtraces

The gdb hooks for debugging CPython (within Tools/gdb) have
been enhanced to show information on more C frames relevant to CPython within
the "py-bt" and "py-bt-full" commands:
  * C frames that are waiting on the GIL
  * C frames that are garbage-collecting
  * C frames that are due to the invocation of a PyCFunction