cmd/link: skip TestFlagW on platforms without DWARF symbol table

cmd/link: skip TestFlagW on platforms without DWARF symbol table

As with other DWARF tests, don't run TestFlagW on platforms
where executables don't have a DWARF symbol table.

Fixes #75585

Change-Id: I81014bf59b15e30ac1b2a7d24a52f9647db46c26
Reviewed-on: https://go-review.googlesource.com/c/go/+/706418
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>