[LLDB][ObjectFileELF] Correct the return type of RelocOffset64 and RelocAddend64

According to `/usr/include/elf.h` and `lldb/source/Plugins/ObjectFile/ELF/ELFHeader.h`.
For ELF64 relocation, types of `offset` and `addend` should be `elf_addr` and `elf_sxword`.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D145550