[dev.link] cmd/internal/obj: add dump of aux symbols for -S=2

[dev.link] cmd/internal/obj: add dump of aux symbols for -S=2

For compiler developers interested in seeing DWARF generation details,
this patch provides symbol "debug asm" dumps for DWARF aux symbols
when -S=2 is in effect.

Change-Id: I5a0b6b65ce7b708948cbbf23c6b0d279bd4f8d9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/223017
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>