We currently sort dynamic relocations by (!is_relative,symbol_index).
Change it to (symbol_index,r_offset). We still place relative
relocations first because we only emit R_*_RELATIVE as dynamic
relocations with 0 symbol index.
This makes readelf -r debugging easier (relocations to the same symbol
are ordered by r_offset).
Refactor the test combreloc.s (renamed from combrelocs.s) to check
R_X86_64_RELATIVE, and delete --expand-relocs.