cmd/link reads PE object files when building programs with cgo.
cmd/link accesses object relocations. Add new Section.Relocs that
provides similar functionality in debug/pe.

Updates #15345

Change-Id: I34de91b7f18cf1c9e4cdb3aedd685486a625ac92
Reviewed-on: https://go-review.googlesource.com/22332
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>