util/mkerr.pl: make it not depend on the function code

util/mkerr.pl: make it not depend on the function code

The output C code was made to use ERR_func_error_string() to see if a
string table was already loaded or not.  Since this function returns
NULL always, this check became useless.

Change it to use ERR_reason_error_string() instead, as there's no
reason to believe we will get rid of reason strings, ever.

To top it off, we rebuild all affected C sources.

Fixes #9756

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9756)