cmake: Detect features at compile time

Instead of relying on manually passed down flags from CMake,
we now have ppsspp_config.h file to create the platform defines for us.
This improves support for multiplatform builds (such as iOS).

Common/MipsCPUDetect.cpp

@@ -158,7 +158,7 @@ void CPUInfo::Detect()
 {
 	// Set some defaults here
 	HTT = false;
-#ifdef _ARCH_64
+#ifdef PPSSPP_ARCH(MIPS64)
 	OS64bit = true;
 	CPU64bit = true;
 	Mode64bit = true;