Metal: Leave the backend out of the libretro cores

They get their graphics context from the frontend, so they compiled and
linked the Metal backend without being able to use it. Adds
PPSSPP_API_METAL, like PPSSPP_API_D3D11, for the code that only exists
with it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/Config.cpp

@@ -609,7 +609,7 @@ bool Config::IsBackendEnabled(GPUBackend backend) {
 		return false;
 #endif
 
-#if !PPSSPP_PLATFORM(MAC) && !PPSSPP_PLATFORM(IOS)
+#if !PPSSPP_API(METAL)
 	if (backend == GPUBackend::METAL)
 		return false;
 #endif