Properly export the various 'use dGPU' symbols when building with MingW

This fixes exporting the NvOptimusEnablement export when building with
MingW. This also adds the equivalent for AMD.

This fixes #23400

platform/windows/os_windows.cpp

@@ -58,11 +58,8 @@
 static const WORD MAX_CONSOLE_LINES = 1500;
 
 extern "C" {
-#ifdef _MSC_VER
-_declspec(dllexport) DWORD NvOptimusEnablement = 0x00000001;
-#else
-__attribute__((visibility("default"))) DWORD NvOptimusEnablement = 0x00000001;
-#endif
+__declspec(dllexport) DWORD NvOptimusEnablement = 1;
+__declspec(dllexport) int AmdPowerXpressRequestHighPerformance = 1;
 }
 
 // Workaround mingw-w64 < 4.0 bug