[dev.link] cmd/internal/obj, cmd/link: use aux symbol for DWARF symbols

[dev.link] cmd/internal/obj, cmd/link: use aux symbol for DWARF symbols

Use the auxiliary symbol mechanism to connect the text symbol and
its associated DWARF symbols. This way, the linker can track the
DWARF symbols from the text symbol, without looking up special
names.

Currently, in the linker this is only used in the deadcode pass
to track which DWARF symbols are used and need to load. Later
passes still use name lookup for now.

Change-Id: I2fe49f3b1f0ecc1472ae8aa93907cff740022d8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/199801
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>