x86: merge put_user.

Move both versions, which are highly similar, to uaccess.h.
Note that, for x86_64, X86_WP_WORKS_OK is always defined.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>