[release-branch.go1.8] cmd/link: disable mach-o dwarf munging with -w (in addition to -s)

Might as well provide a way around the mach-o munging
that doesn't require stripping all symbols.
After all, -w does mean no DWARF.

For #11887, #19734, and anyone else that needs to disable
this code path without losing the symbol table.

Change-Id: I254b7539f97fb9211fa90f446264b383e7f3980f
Reviewed-on: https://go-review.googlesource.com/39602
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -1223,7 +1223,7 @@ func (l *Link) hostlink() {
 		l.Logf("%s", out)
 	}
 
-	if !*FlagS && !debug_s && Headtype == obj.Hdarwin {
+	if !*FlagS && !*FlagW && !debug_s && Headtype == obj.Hdarwin {
 		// Skip combining dwarf on arm.
 		if !SysArch.InFamily(sys.ARM, sys.ARM64) {
 			dsym := filepath.Join(*flagTmpdir, "go.dwarf")