src: clean up large pages code

* Initialize structure members.
* Factor out common `Debug()` prefix.
* Remove unused `text_region` field.

Signed-off-by: Gabriel Schulhof <gabriel.schulhof@intel.com>
PR-URL: https://github.com/nodejs/node/pull/33255


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>