Suppress -Winvalid-offsetof for PowerPC::PowerPCState

This code doesn't need to be portable (since the goal is to have a smaller offset for x64 codegen), so if it's not supported there are other problems. Similar code exists in e.g. DSP.cpp.

Source/Core/Core/PowerPC/PowerPC.h

@@ -223,8 +223,15 @@ struct PowerPCState
 };
 
 #if _M_X86_64
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Winvalid-offsetof"
+#endif
 static_assert(offsetof(PowerPC::PowerPCState, above_fits_in_first_0x100) <= 0x100,
               "top of PowerPCState too big");
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
 #endif
 
 extern PowerPCState ppcState;