[ELF] Use parallelSort for .rela.dyn

An unstable sort suffices. In a large link (11.06s), this decreases .rela.dyn
writeTo time from 1.52s to 0.81s, resulting in 6% total time speedup (the
benefit will greatly dilute if --pack-dyn-relocs=relr becomes prevailing).

Encoding the dynamic relocations then sorting raw Elf_Rel/Elf_Rela doesn't seem
to improve much (doing that would require code duplicate because of
Elf_Rel/Elf_Rela plus unfortunate mips64le), so don't do that.