x86: put movsl_mask into uaccess.h.

x86_64 does not need it, but it won't have X86_INTEL_USERCOPY
defined either.

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>