[dev.link] cmd/link: keep DWARF constant DIE symbols live

[dev.link] cmd/link: keep DWARF constant DIE symbols live

DWARF constant DIE symbols are not referenced by any other symbol,
but are needed by the DWARF pass, where they get linked to the
compilation unit.

Reenable gdb constant test.

Change-Id: If77a0d379d9a6f1591939345bc31b027c2567f22
Reviewed-on: https://go-review.googlesource.com/c/go/+/204397
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>