CPU: Make fastmem a compile-time feature (support 32-bit targets)

src/core/cpu_recompiler_thunks.h

@@ -32,8 +32,12 @@ void UncheckedWriteMemoryByte(u32 address, u8 value);
 void UncheckedWriteMemoryHalfWord(u32 address, u16 value);
 void UncheckedWriteMemoryWord(u32 address, u32 value);
 
+#ifdef WITH_FASTMEM
+
 void UpdateFastmemMapping();
 
+#endif
+
 } // namespace Recompiler::Thunks
 
-} // namespace CPU
+} // namespace CPU