Here is a patch to add a default command debugger-trap to break into
GDB, together with a default breakpoint set in .gdbinit
The immediate benefit for developers is a standard command to use where
there isn't one at the moment, as explained in DEBUG, only general guidance.

The long-term benefit is it can be the foundation for further work to
enhance debugging in Emacs with GDB, with the ability to
programmatically break into GDB, something I am working on as part of
C/Lisp hybrid debugging aids.


This patch has received substantial advice and reviewed by Eli.  It is
posted here for the benefit of the community for further discussion.

To recap, we need a do-nothing primitive, whose sole purpose is to
allow setting a breakpoint there to get control to GDB.
redraw-display and similar functions are usually okay, but they could
 get in the way if what they do modifies Emacs in a way that interferes
with debugging, for example if they remove some display artifact one
is debugging, or run functions as side effect that modify the global
state.


Adding Paul Eggert as the main recent contributor to etc/DEBUG for thoughts.