[common] fallback to VirtualBuffer if we can't use file mappings (#4005)

fixes:
- windows 8.1
- windows 7
- managarm
- openorbis

Signed-off-by: lizzie lizzie@eden-emu.dev

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4005
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Reviewed-by: Maufeat <sahyno1996@gmail.com>

src/common/host_memory.h

@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: Copyright 2025 Eden Emulator Project
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 // SPDX-FileCopyrightText: Copyright 2019 yuzu Emulator Project
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 #include "common/common_funcs.h"
 #include "common/common_types.h"
 #include "common/virtual_buffer.h"
@@ -76,12 +77,16 @@ private:
     size_t backing_size{};
     size_t virtual_size{};
 
+#if !(defined(__OPENORBIS__) || defined(__managarm__))
     // Low level handler for the platform dependent memory routines
     class Impl;
     std::unique_ptr<Impl> impl;
+#endif
     u8* backing_base{};
     u8* virtual_base{};
     size_t virtual_base_offset{};
+    // Windows requires it for kernels whom lack proper support for some functions!
+    std::optional<Common::VirtualBuffer<u8>> fallback_buffer;
 };
 
 } // namespace Common