[ELF] --gc-sections: allow GC on reserved sections in a group

This generalizes D70146 (SHT_NOTE) to more reserved sections and makes our rules
more consistent. Now SHF_GROUP is more similar to SHF_LINK_ORDER.

For SHT_INIT_ARRAY/SHT_FINI_ARRAY, the rule will be closer to PE/COFF link.exe.

Previously sanitizers use llvm.global_ctors to make module_ctor a GC
root, which is considered an abuse.
https://groups.google.com/g/generic-abi/c/TpleUEkNoQI

We can squeak through on compatibility issues because compilers otherwise don't
use SHF_GROUP special sections.