Add a common Kconfig CONFIG_ARCH_HAS_UACCESS_MCSAFE that archs can
optionally select, and fixup the declaration of _copy_to_iter_mcsafe().

Fixes: 8780356e ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Signed-off-by: NDan Williams <dan.j.williams@intel.com>