MINOR: debug: add struct ha_caller to describe a calling location

MINOR: debug: add struct ha_caller to describe a calling location

The purpose of this structure is to assemble all constant parts of a
generic calling point for a specific event. These ones are created by
the compiler as a static const element outside of the code path, so
they cost nothing in terms of CPU, and a pointer to that descriptor
can be passed to the place that needs it. This is very similar to what
is being done for the mem_stat stuff.

This will be useful to simplify and improve DEBUG_TASK.