Build fixes for rtla

The rtla build currently has a number of problems:

• It makes a copy of the upstream source, which is unnecessary since out-of-tree builds are now (mostly) supported since Linux 6.9. This also resulted in needing to patch out a use of 'make kernelversion' in the upstream Makefile that is otherwise harmless.
• It's not verbose, so the buildflags are not being checked by blhc.
• The buildflags from dpkg are no longer correctly passed through to the upstream Makefile, since Linux 6.9.
• Debug symbols are missing (originally fixed by !741 (merged)).
• It builds with LTO for no obvious reason (originally fixed by !741 (merged) as a problem for reproducibility).

Fix all of these.