cmd/compile: skip TestStmtLines on Plan 9

TestStmtLines has been added in CL 102435.
This test is failing on Plan 9 because executables
don't have a DWARF symbol table.

Fixes #25387.

Change-Id: I6ae7cba0e8ad4ab569a29ea8920b7849acfb9846
Reviewed-on: https://go-review.googlesource.com/113115
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/stmtlines_test.go

@@ -56,6 +56,10 @@ func (f *File) Get(lineno int) (string, bool) {
 }
 
 func TestStmtLines(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	lines := map[Line]bool{}
 	dw, err := open(testenv.GoToolPath(t))
 	must(err)