[dev.link] cmd/link: directly use loader.ExtReloc in ELF relocation generation

[dev.link] cmd/link: directly use loader.ExtReloc in ELF relocation generation

Convert the part that uses relocations to use loader.ExtReloc
directly. It still uses sym.Symbols for now, but not sym.Relocs.

This reduces some memory usage: linking cmd/compile with external
linking,

name             old allocs/op  new allocs/op  delta
Loadlibfull_GC     52.2MB ± 0%    13.9MB ± 0%  -73.40%  (p=0.008 n=5+5)

name             old live-B     new live-B     delta
Loadlibfull_GC      75.5M ± 0%     61.9M ± 0%  -18.02%  (p=0.008 n=5+5)

Change-Id: I317ecbf516063c42b255b2caba310ea6281342d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/231319
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>