test: fix fixedbugs/issue30908.go to work with no-opt builder

Update the issue 30908 test to work with the no-opt builder
(this requires a corresponding change in the linker as well).
As part of this change, 'rundir' tests are now linked without
passing "-w" to the linker.

Updates #30908.
Fixes #31034.

Change-Id: Ic776e1607075c295e409e1c8230aaf55a79a6323
Reviewed-on: https://go-review.googlesource.com/c/go/+/169161Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue30908.go

-// rundir -P -l=4 -ldflags -strictdups=2
+// rundir -P -ldflags -strictdups=2 -w=0
 
 // Copyright 2019 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style