[dev.link] cmd/link/internal/objfile: new loader method for reloc queries

[dev.link] cmd/link/internal/objfile: new loader method for reloc queries

Second change of several to update the loader API to reflect the final
consensus version of the loader API as described in Cherry's doc.
This piece:

 - define new loader.Relocs() method that returns a struct
   encapsulating a set of relocations on a global symbol

Old way of examining relocations:

  nreloc := loader.NReloc(someGlobalSymbolIndex)
  for i := 0; i < nreloc; i++ {
    tgtIdx := loader.RelocSym(someGlobalSymbolIndex, i))
    ... <do something with tgtIdx>
  }

New way of examining relocations:

  relocs := d.loader.Relocs(someGlobalSymbolIndex)
  for i := 0; i < relocs.Count; i++ {
    r := relocs.At(i).Sym
    ... <do something with r.Sym>
  }

Change-Id: I5bead1d729655ea13b3396647e53aafcd3e60f97
Reviewed-on: https://go-review.googlesource.com/c/go/+/200717
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>