debug/gosym: fix TestPCLine on Plan 9

Plan 9 doesn't define main, so the INITENTRY
symbol remains with the SXREF type, which leads
Entryvalue to fail on "entry not text: main".

Fixes #14536.

Change-Id: Id9b7d61e5c2202aba3ec9cd52f5b56e0a38f7c47
Reviewed-on: https://go-review.googlesource.com/19973
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/gosym/pclntab_test.go

@@ -42,7 +42,7 @@ func dotest(t *testing.T) {
 	if err := cmd.Run(); err != nil {
 		t.Fatal(err)
 	}
-	cmd = exec.Command("go", "tool", "link", "-H", "linux", "-E", "main",
+	cmd = exec.Command("go", "tool", "link", "-H", "linux",
 		"-o", pclinetestBinary, pclinetestBinary+".o")
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr