common/host_memory: Implement a fallback if fastmem fails.

This falls back to the old approach of using a virtual buffer.

Windows is untested, but this build should fix support for Windows < 10 v1803. However without fastmem support at all.

src/common/host_memory.h

@@ -6,6 +6,7 @@
 
 #include <memory>
 #include "common/common_types.h"
+#include "common/virtual_buffer.h"
 
 namespace Common {
 
@@ -61,6 +62,9 @@ private:
     u8* backing_base{};
     u8* virtual_base{};
     size_t virtual_base_offset{};
+
+    // Fallback if fastmem is not supported on this platform
+    std::unique_ptr<Common::VirtualBuffer<u8>> fallback_buffer;
 };
 
 } // namespace Common