cmd/link: relocate dwarf examiner helper to separate package

The linker DWARF test includes an "examiner" helper type (with
associated methods) that is used to help linker DWARF tests read DWARF
info in a higher level and more structured way than just raw
debug/dwarf operations. This patch extracts out "examiner" and
relocates it to a separate package, so that it can be used in other
package tests as well, if need be.

Change-Id: Iec66061e2719ee698c12d8fa17b11698442b336d
Reviewed-on: https://go-review.googlesource.com/c/go/+/364036


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>