os: add stubs for Readlink and File.Seek for baremetal targets, with smoke test.

Test currently enabled on pybadge (chosen at random)

TODO:
- enable test on arduino; currently fails with "interp: ptrtoint integer size..." (#2389)
- enable test on nintendoswitch; currently fails with many missing definitions (#2530)

tests/os/smoke/smoke_test.go

@@ -0,0 +1,27 @@
+package os_smoke_test
+
+// Simple smoke tests for the os package or things that depend on it.
+// Intended to catch build tag mistakes affecting bare metal targets.
+
+import (
+	"fmt"
+	"path/filepath"
+	"testing"
+)
+
+// Regression test for https://github.com/tinygo-org/tinygo/issues/2563
+func TestFilepath(t *testing.T) {
+	if filepath.Base("foo/bar") != "bar" {
+		t.Errorf("filepath.Base is very confused")
+	}
+}
+
+// Regression test for https://github.com/tinygo-org/tinygo/issues/2530
+func TestFmt(t *testing.T) {
+	n, err := fmt.Printf("Hello, world!\n")
+	if err != nil {
+		t.Errorf("printf returned error %s", err)
+	} else if n != 14 {
+		t.Errorf("printf returned %d, expected 14", n)
+	}
+}