cmd/link/internal/ld: run tests in parallel

Also skip TestNooptCgoBuild in short mode.

Also fix a couple of obscure constants to use values named in
cmd/internal/dwarf.

This brings the time of the cmd/link/internal/ld tests down to about 1
second on my laptop.

Updates #26470

Change-Id: I71c896f30fd314a81d9090f1b6d02edc4174a808
Reviewed-on: https://go-review.googlesource.com/c/153259


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>