runtime: add several os package stubs

src/os/file_other.go

@@ -8,7 +8,7 @@ import (
 
 // Read is unsupported on this system.
 func (f *File) Read(b []byte) (n int, err error) {
-	return 0, ErrUnsupported
+	return 0, errUnsupported
 }
 
 // Write writes len(b) bytes to the output. It returns the number of bytes
@@ -21,13 +21,13 @@ func (f *File) Write(b []byte) (n int, err error) {
 		}
 		return len(b), nil
 	default:
-		return 0, ErrUnsupported
+		return 0, errUnsupported
 	}
 }
 
 // Close is unsupported on this system.
 func (f *File) Close() error {
-	return ErrUnsupported
+	return errUnsupported
 }
 
 //go:linkname putchar runtime.putchar