Unify fastmem_pointer handling by using reinterpret_cast<uintptr_t>()
and std::nullopt consistently across all platforms, fixing MSVC
compilation errors with std::optional<uintptr_t> assignment.
Fixes: C2679 binary '=' operator errors on MSVC
Signed-off-by: Zephyron <zephyron@citron-emu.org>