os: add basic OS functionality

src/os/file_wasm.go

@@ -0,0 +1,34 @@
+// +build wasm
+
+package os
+
+import (
+	_ "unsafe"
+)
+
+// Read is unsupported on this system.
+func (f *File) Read(b []byte) (n int, err error) {
+	return 0, ErrUnsupported
+}
+
+// Write writes len(b) bytes to the output. It returns the number of bytes
+// written or an error if this file is not stdout or stderr.
+func (f *File) Write(b []byte) (n int, err error) {
+	switch f.fd {
+	case Stdout.fd, Stderr.fd:
+		for _, c := range b {
+			putchar(c)
+		}
+		return len(b), nil
+	default:
+		return 0, ErrUnsupported
+	}
+}
+
+// Close is unsupported on this system.
+func (f *File) Close() error {
+	return ErrUnsupported
+}
+
+//go:linkname putchar runtime.putchar
+func putchar(c byte)