tile: add <asm/word-at-a-time.h> and enable support functions

tile: add <asm/word-at-a-time.h> and enable support functions

commit 5bf6c07a1843813d0065feaaecba622d49148d7e upstream.

This change enables the generic strncpy_from_user() and strnlen_user()
using word-at-a-time.h.  The tile implementation is trivial since
both tilepro and tilegx have SIMD operations that do byte-wise
comparisons against immediate zero for each byte, and return an
0x01 byte in each position where there is a 0x00 byte.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>