Enable zstd-compressed debug symbols by default

[ReillyBrogan]

Now that LLVM 16 has landed the last blocker to zstd-compressed debug symbols has been removed. Adding -gz=zstd to the CFLAGS/CXXFLAGS might be sufficient, needs more investigation. We also need to ensure that there is a toggle to turn this off, mostly for when building with LLVM 15 (although I kinda want to see if backporting support is viable, the commits don't seem particularly big) or some other older compiler.