Im trying to reduce memory used by
internal tables built into DYNAMIC_DEBUG

the main savings available is in the per pr_debug*
callsite data: modname, filename, funcname.

I have segregated those fields to a new __dyndbg_sites section,
described by struct _ddebug_site, and now refd by new ptr in
struct _ddebug

That new ptr enlarges the memory footprint, so I want to replace it
with an integer index into the section / array.  Existing padding could
hold the index.

To use the index, I need  &dyndbg_sites[], and that only works
for builtin-module's callsites.   For loaded modules, I can/have
added a pointer to the section into module load_info, giving me
the base I will need for the ! builtin branch.

I just need a not expensive  is-it-builtin (modref)