Make generated files read-only

Make generated files read-only

This is an attempt to discouraged manual changes of generated files,
as people have done so, just to get their changes over-written next
time those files are re-generated.

Reference: https://github.com/openssl/openssl/discussions/28269

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28612)