[ELF] Support relocatable files using CREL with explicit addends ... using the temporary section type code 0x40000020 (`clang -c -Wa,--crel,--allow-experimental-crel`). LLVM will change the code and break compatibility (Clang and lld of different versions are not guaranteed to cooperate, unlike other features). CREL with implicit addends are not supported. --- Introduce `RelsOrRelas::crels` to iterate over SHT_CREL sections and update users to check `crels`. (The decoding performance is critical and error checking is difficult. Follow `skipLeb` and `R_*LEB128` handling, do not use `llvm::decodeULEB128`, whichs compiles to a lot of code.) A few users (e.g. .eh_frame, LLDDwarfObj, s390x) require random access. Pass `/*supportsCrel=*/false` to `relsOrRelas` to allocate a buffer and convert CREL to RELA (`relas` instead of `crels` will be used). Since allocating a buffer increases, the conversion is only performed when absolutely necessary. --- Non-alloc SHT_CREL sections may be created in -r and --emit-relocs links. SHT_CREL and SHT_RELA components need reencoding since r_offset/r_symidx/r_type/r_addend may change. (r_type may change because relocations referencing a symbol in a discarded section are converted to `R_*_NONE`). * SHT_CREL components: decode with `RelsOrRelas` and re-encode (`OutputSection::finalizeNonAllocCrel`) * SHT_RELA components: convert to CREL (`relToCrel`). An output section can only have one relocation section. * SHT_REL components: print an error for now. SHT_REL to SHT_CREL conversion for -r/--emit-relocs is complex and unsupported yet. Link: https://discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600 Pull Request: https://github.com/llvm/llvm-project/pull/98115
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