[dev.link] cmd/link: add methods for adding relocations in Reloc2 format

This is in prepration of removing the old loader.Reloc. This also
introduces a way of adding a slice of relocations more
efficiently (will be used in the next CL).

Change-Id: I3eaee7fb3a3e102a8670990f4a31c40d0b17b8c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/227761


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