In commit 49e2c8f0 (MDEV-25743)
we made dict_sys_t::find() incompatible with the rest of the
table name hash table operations in case the table name contains
non-ASCII octets (using a compatibility mode that facilitates the
upgrade into the MySQL 5.0 filename-safe encoding) and the target
platform implements signed char.

ut_fold_string(): Remove; replace with my_crc32c(). This also makes
table name hash value calculations independent on whether char
is unsigned or signed.