[ELF] Optimize RelocationSection<ELFT>::writeTo

When linking a 1.2G output (nearly no debug info, 2846621 dynamic relocations) using `--threads=8`, I measured

```
9.131462 Total ExecuteLinker
1.449913 Total Write output file
1.445784 Total Write sections
0.657152 Write sections {"detail":".rela.dyn"}
```

This change decreases the .rela.dyn time to 0.25, leading to 4% speed up in the total time.

* The parallelSort is slow because of expensive r_sym/r_offset computation. Cache the values.
* The iteration is slow. Move r_sym/r_addend computation ahead of time and parallelize it.

With the change, the new encodeDynamicReloc is cheap (0.05s). So no need to parallelize it.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D115993