databases/mysql84-{client,server}: Fix MySQL Router plugin loading

MySQL Router fails to start when the configuration contains an
[http_server] section, as generated by mysqlsh:

Error: Loading plugin for config-section '[http_server]' failed:
libmysqlrouter_http_server.so.1: No space available for static
Thread Local Storage

Upstream compiles with -ftls-model=initial-exec, so router libraries
loaded through dlopen(3) have to be placed in the static TLS block.
libmysqlrouter_http_server.so.1 needs 272 bytes and libmysqlharness.so.1
another 32, while rtld reserves 128 bytes for objects loaded later.
The five rest_* plugins depend on http_server and fail along with it.

PR: 298483
Reported by: marko.cupac@mimar.rs
Sponsored by: Netzkommune GmbH