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>

test/functional/lua/vim_spec.lua

@@ -763,6 +763,20 @@ describe('lua stdlib', function()
       pcall_err(exec_lua, code))
   end)
 
+  it('vim.spairs', function()
+    local res = ''
+    local table = {
+      ccc=1,
+      bbb=2,
+      ddd=3,
+      aaa=4
+    }
+    for key, _ in vim.spairs(table) do
+      res = res .. key
+    end
+    matches('aaabbbcccddd', res)
+  end)
+
   it('vim.call, vim.fn', function()
     eq(true, exec_lua([[return vim.call('sin', 0.0) == 0.0 ]]))
     eq(true, exec_lua([[return vim.fn.sin(0.0) == 0.0 ]]))