MySQL 5.7 supports only one shared temporary tablespace.

MariaDB 10.2 does not support any other shared InnoDB tablespaces than
the two predefined tablespaces: the persistent InnoDB system tablespace
(default file name ibdata1) and the temporary tablespace
(default file name ibtmp1).

InnoDB is unnecessarily allocating a tablespace ID for the predefined
temporary tablespace on every startup, and it is in several places
testing whether a tablespace ID matches this dynamically generated ID.

We should use a compile-time constant to reduce code size and to avoid
unnecessary updates to the DICT_HDR page at every startup.

Using a hard-coded tablespace ID will should make it easier to remove the
TEMPORARY flag from FSP_SPACE_FLAGS in MDEV-11202.