build: make generated source files reproducible #21586

Problem:
Build is not reproducible, because generated source files (.c/.h/) are not
deterministic, mostly because Lua pairs() is unordered by design (for security).

https://github.com/LuaJIT/LuaJIT/issues/626#issuecomment-707005671
https://www.lua.org/manual/5.1/manual.html#pdf-next
> The order in which the indices are enumerated is not specified [...]
>
>> The hardening of the VM deliberately randomizes string hashes. This in
>> turn randomizes the iteration order of tables with string keys.

Solution:
- Update the code generation scripts to be deterministic.
    - That is only a partial solution: the exported function
      (funcs_metadata.generated.h) and ui event
      (ui_events_metadata.generated.h) metadata have some mpack'ed
      tables, which are not serialized deterministically.
    - As a workaround, introduce `PRG_GEN_LUA` cmake setting, so you can
      inject a modified build of luajit (with LUAJIT_SECURITY_PRN=0)
      that preserves table order.
    - Longer-term we should change the mpack'ed data structure so it no
      longer uses tables keyed by strings.

Closes #20124

Co-Authored-By: dundargoc <gocdundar@gmail.com>
Co-Authored-By: Arnout Engelen <arnout@bzzt.net>

src/nvim/generators/gen_api_dispatch.lua

@@ -426,7 +426,9 @@ for _,fn in ipairs(functions) do
 end
 remote_fns.redraw = {impl_name="ui_client_redraw", fast=true}
 
-local hashorder, hashfun = hashy.hashy_hash("msgpack_rpc_get_handler_for", vim.tbl_keys(remote_fns), function (idx)
+local names = vim.tbl_keys(remote_fns)
+table.sort(names)
+local hashorder, hashfun = hashy.hashy_hash("msgpack_rpc_get_handler_for", names, function (idx)
   return "method_handlers["..idx.."].name"
 end)