This is a refactor to pave the way for supporting paired-ADDEND for ARM64. The only paired reloc type for X86_64 is SUBTRACTOR. In a later diff, I will add SUBTRACTOR for both X86_64 and ARM64.
- s/getImplicitAddend/getAddend/ because it handles all forms of addend: implicit, explicit, paired.
- add predicate bool isPairedReloc()
- check range of relInfo.r_symbolnum is internal, unrelated to user-input, so use assert(), not error()
- minor cleanups & rearrangements in InputFile::parseRelocations()