runtime/pprof: add GNU build IDs to Mappings recorded from /proc/self/maps

runtime/pprof: add GNU build IDs to Mappings recorded from /proc/self/maps

This helps systems that maintain an external database mapping
build ID to symbol information for the given binary, especially
in the case where /proc/self/maps lists many different files
(for example, many shared libraries).

Avoid importing debug/elf to avoid dragging in that whole
package (and its dependencies like debug/dwarf) into the
build of every program that generates a profile.

Fixes #19431.

Change-Id: I6d4362a79fe23e4f1726dffb0661d20bb57f766f
Reviewed-on: https://go-review.googlesource.com/37855
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>