MariaDB is compiled with C++ exceptions enabled, and that disallows
some optimizations (e.g. the stack must always be unwinding-safe).  By
adding `noexcept` to functions that are guaranteed to never throw,
some of these optimizations can be regained.  Low-level locking
functions that are called often are a good candidate for this.

This shrinks the executable a bit (tested with GCC 14 on aarch64):

    text	  data	   bss	   dec	   hex	filename
 24448910	2436488	9473185	36358583	22ac9b7	build/release/sql/mariadbd
 24448622	2436488	9473537	36358647	22ac9f7	build/release/sql/mariadbd

Another chance for cutting back overhead due to C++ exceptions being
enabled; the `dict_sys_t` class is a good candidate because its
locking methods are called frequently.

Binary size reduction this time:

    text	  data	   bss	   dec	   hex	filename
 24448622	2436488	9473537	36358647	22ac9f7	build/release/sql/mariadbd
 24448474	2436488	9473601	36358563	22ac9a3	build/release/sql/mariadbd