[dev.link] cmd/link: check for dwarf enabled in loadlibfull

[dev.link] cmd/link: check for dwarf enabled in loadlibfull

Check dwarfEnabled before invoking dwarfConvertSymbols (not needed if
we're not doing dwarf gen).

Change-Id: Id7ea7d11c13524705d305596bf1468d4858216b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/222157
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>