Specify '-optimize-size' when building Qt6

This is a part of effort to further reduce Qt binary sizes (#822).

As far as Qt binaries are concerned, we would prefer binary sizes over
performance as these binaries are not used so often.  This is rather
important for universal binary build in macOS, where we literally
bundle two binaries.

This commit specify '-optimize-size' [1] when configuring Qt6.  The
overall results look OK-ish in macOS and rather impressive in Windows.

 macOS aarch64 release build
  QtCore:         6545752 -> 6354056 =  -191696
  QtGui:          8609112 -> 8237560 =  -371552
  QtWidgets:      7003272 -> 7136344 =  +133072
                                        -430176

 Windows x64_64 release build
  Qt6Core.dll:    5982720 -> 5190656 =  -792064
  Qt6Gui.dll:     6276096 -> 4994048 = -1282048
  Qt6Widgets.dll: 6099456 -> 5074432 = -1025024
                                       -3099136

As '-optimize-size' is something that is officially supported in Qt,
there is expected to be no observable behavior change.

 [1]: https://doc.qt.io/qt-6/configure-options.html#debug-and-release-builds

PiperOrigin-RevId: 573731405

src/build_tools/build_qt.py

@@ -262,6 +262,7 @@ def make_configure_options(args: argparse.Namespace) -> list[str]:
 
   qt_configure_options = ['-opensource',
                           '-c++std', 'c++20',
+                          '-optimize-size',
                           '-silent',
                           '-no-cups',
                           '-no-dbus',