[dev.link] cmd/link: use new-style relocation accessors in LoadFull

This gives some small speedup:

(linking cmd/compile)
Loadlibfull     77.8ms ± 6%    68.6ms ± 5%  -11.88%  (p=0.008 n=5+5)

Also convert some Relocs.At to At2, which should have been done
earlier.

Change-Id: I2a66aeb5857234c6e645e1b23380149cffc8221f
Reviewed-on: https://go-review.googlesource.com/c/go/+/226363


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>