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_writeout.c

@@ -580,7 +580,7 @@ static const char *outtime(const char *ptr, /* %time{ ... */
     if(!result) {
       struct tm utc;
       result = curlx_gmtime(secs, &utc);
-#ifdef __GNUC__
+#ifdef __GNUC__  /* includes llvm/clang, but not affected as of v22.1.0 */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wformat-nonliteral"
 #endif