build: compiler warning silencing tidy-ups

- tool_getparam: revert an unnecessary/no-op C89 warning silencer.
  Follow-up to 09c9afdd71 #20363

- tool_writeout: add comment saying silencing is a no-op for llvm/clang.
  For `strftime()` it is a GCC-specific, as of llvm/clang v22.1.0.
  Follow-up to f07a98ae11 #20366

- unit1652: drop always-false `!defined(__clang__)` guard.
  Pointed-out-by: Orgad Shaneh
  Ref: #20902
  Follow-up to 7e814c8717 #16062

- unit1652: document that `-Wformat` is necessary for GCC v5 to v8.
  Follow-up to 71cf0d1fca #14772

Closes #20908

src/tool_getparam.c

@@ -3057,7 +3057,8 @@ ParameterError parse_args(int argc, argv_item_t argv[])
   ParameterError err = PARAM_OK;
   struct OperationConfig *config = global->first;
 
-  for(i = 1, stillflags = TRUE; i < argc && !err; i++) {
+  stillflags = TRUE;
+  for(i = 1; i < argc && !err; i++) {
     orig_opt = convert_tchar_to_UTF8(argv[i]);
     if(!orig_opt)
       return PARAM_NO_MEM;