[dev.link] cmd/internal/goobj2, cmd/link: experiment another way of accessing relocations

Use a different mechanism to access relocations from the object
files, and use it in the stack bounds check pass. This shows some
speedup.

(linking cmd/compile)
Dostkcheck     76.9ms ± 1%    55.1ms ± 1%  -28.36%  (p=0.008 n=5+5)

Change-Id: I2ac42da515dccd64719fb557ffff6cdc69e4319b
Reviewed-on: https://go-review.googlesource.com/c/go/+/222240
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/loader/loader.go

@@ -49,6 +49,17 @@ type Reloc struct {
 	Sym  Sym              // global index of symbol the reloc addresses
 }
 
+// Reloc2 holds a "handle" to access a relocation record from an
+// object file.
+type Reloc2 struct {
+	*goobj2.Reloc2
+	r *oReader
+	l *Loader
+}
+
+func (rel Reloc2) Type() objabi.RelocType { return objabi.RelocType(rel.Reloc2.Type()) }
+func (rel Reloc2) Sym() Sym               { return rel.l.resolve(rel.r, rel.Reloc2.Sym()) }
+
 // oReader is a wrapper type of obj.Reader, along with some
 // extra information.
 // TODO: rename to objReader once the old one is gone?
@@ -1435,6 +1446,16 @@ func (relocs *Relocs) At(j int) Reloc {
 	}
 }
 
+func (relocs *Relocs) At2(j int) Reloc2 {
+	if relocs.l.isExtReader(relocs.r) {
+		// TODO: implement this. How? Maybe we can construct the reloc
+		// data for external symbols in the same byte form as the one
+		// in the object file?
+		panic("not implemented")
+	}
+	return Reloc2{relocs.r.Reloc2(relocs.li, j), relocs.r, relocs.l}
+}
+
 // ReadAll method reads all relocations for a symbol into the
 // specified slice. If the slice capacity is not large enough, a new
 // larger slice will be allocated. Final slice is returned.