refactor: move x86 uaccess page table walking to src/arch/x86/uaccess.c

refactor: move x86 uaccess page table walking to src/arch/x86/uaccess.c

The uaccess implementation used x86-specific recursive page table
mapping (0xFFFFF000/0xFFC00000) for user_range_ok, copy_from_user,
copy_to_user, and uaccess_try_recover. This is 100% arch-dependent.

- Move full x86 implementation to src/arch/x86/uaccess.c (no guards)
- Replace src/kernel/uaccess.c with weak stubs (simple memcpy-based)
- The linker picks the arch-specific version when available

Passes: make, cppcheck, QEMU smoke test.