fix(generator): Define __x86_64__ to support Qt SDK on multi-lib distribution

This ensures that headers like `bits/wordsize.h` included from `qconfig.h`
found in multi-lib Qt distributions will properly define `__WORDSIZE` so that
the expected header `QtCore/qconfig-64.h` is included.

Author: jcfr

generator/main.cpp

@@ -227,6 +227,9 @@ namespace
       }
       dui.defines.push_back("__cplusplus=1");
       dui.defines.push_back("__STDC__");
+#ifdef Q_PROCESSOR_X86_64
+      dui.defines.push_back("__x86_64__");
+#endif
       dui.std = "c++20";
       dui.removeComments = true;