Add -gsplit-dwarf to LTO build to cut memory utilization in linker

[kevingranade]

This will break debugging for this build for now, but it's currently failing so ehhhh...

Summary

None

Purpose of change

Some chatter on discord brought some build failures like https://github.com/CleverRaven/Cataclysm-DDA/actions/runs/9078930225/job/24950004752#step:14:746 to my attention, looks like memory exhaustion?
Attempted to avoid overlapping linker invocations in #73793 but that didn't do it.

Describe the solution

This passes a flag to the compiler (in github LTO builds only) to output debugging data to a separate file instead of to the .o file.
As a result, at link time the linker doesn't have to load and process that (massive) debug data and doesn't consume all the memory on the system.

Describe alternatives you've considered

There are various overhauls we could look into, but the debug data really is absolutely dominating memoy usage, so I think this is the way to go.

Testing

I ran the build locally using the same build arguments and a similar gcc version (mine: g++ (Ubuntu 9.4.0-1ubuntu120.04.2) 9.4.0, CI: g++-9 (Ubuntu 9.5.0-1ubuntu122.04) 9.5.0) as in the failing CI job.
Before this change:
39.46user 12.12system 1:08.21elapsed 75%CPU (0avgtext+0avgdata 9189096maxresident)k

After this change:
7.80user 1.81system 0:09.89elapsed 97%CPU (0avgtext+0avgdata 1950676maxresident)k

The final test will be if the CI job breaks, but I'm very optomistic.

Additional context

This will break debugging for now since we aren't archiving and storing the debug data, but I think that's a better state to be in temporally than all the builds failing...
This article outlines what the whole process looks like: https://www.tweag.io/blog/2023-11-23-debug-fission/
Once we have that working, I suspect we'll want to switch all the CI builds over to this.

[kevingranade]

Noticed a bug, this does not ONLY get added to LTO builds as I had intended, it should still help diagnose whether this approach will fix the GCC 9 LTO build.

[kevingranade]

After the latest change this no longer supplies the flag to the BasicBuild job at least.