cmd/link: use filepath.Join rather than d + "/" + f

Fixes #26917

Change-Id: I676f016ed43aaa523b6d3a87b28a1d1d2ebe72c4
Reviewed-on: https://go-review.googlesource.com/c/153237
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -136,13 +136,13 @@ func findlib(ctxt *Link, lib string) (string, bool) {
 			// try dot, -L "libdir", and then goroot.
 			for _, dir := range ctxt.Libdir {
 				if ctxt.linkShared {
-					pname = dir + "/" + pkg + ".shlibname"
+					pname = filepath.Join(dir, pkg+".shlibname")
 					if _, err := os.Stat(pname); err == nil {
 						isshlib = true
 						break
 					}
 				}
-				pname = dir + "/" + name
+				pname = filepath.Join(dir, name)
 				if _, err := os.Stat(pname); err == nil {
 					break
 				}