gcc: guard #pragma diagnostic in core code for <4.6, disable picky warnings

Extend `#pragma diagnostic push`/`pop` guards to the whole codebase
(from tests and examples only) to disable it for GCC <4.6. Rename guard
to `CURL_HAVE_DIAG` and make it include llvm/clang to be interchangeable
with `__GNUC__ || __clang__` in this context.

The above means no longer disabling certain warnings locally, so pair
this with disabling all picky warnings for GCC <4.6.

Also:
- drop global workarounds for misbehaving GCC <4.6 compiler warnings.
  Not needed with picky warnings disabled.

Reported-by: fds242 on github
Reported-by: Sergey Fedorov
Thanks-to: Orgad Shaneh
Follow-up to f07a98ae11 #20366
Fixes #20892
Fixes #20924
Closes #20902
Closes #20907

src/tool_writeout.c

@@ -580,14 +580,14 @@ static const char *outtime(const char *ptr, /* %time{ ... */
     if(!result) {
       struct tm utc;
       result = curlx_gmtime(secs, &utc);
-#ifdef __GNUC__  /* includes llvm/clang, but not affected as of v22.1.0 */
+#ifdef CURL_HAVE_DIAG /* includes llvm/clang, but not affected as of v22.1.0 */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wformat-nonliteral"
 #endif
       if(curlx_dyn_len(&format) && !result &&
          strftime(output, sizeof(output), curlx_dyn_ptr(&format), &utc))
         fputs(output, stream);
-#ifdef __GNUC__
+#ifdef CURL_HAVE_DIAG
 #pragma GCC diagnostic pop
 #endif
       curlx_dyn_free(&format);