patch 9.1.1692: global_functions are not constant

patch 9.1.1692: global_functions are not constant

Problem:  global_functions are not constant
Solution: Place global_functions[] in read-only memory (Damien Lejay).

Mark global_functions[] as `static const`.  The table is never modified
at runtime, so keeping it in writable `.data` has no benefit.

Only a local pointer in func_check_arg_types() needed adjusting to
`const`.  No functional changes.

closes: #18121

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>