alpha: extend memset16 to EV6 optimised routines

Commit 92ce4c3e, "alpha: add support for memset16", renamed
the function memsetw() to be memset16() but neglected to do this for
the EV6 optimised version, thus when building a kernel optimised
for EV6 (or later) link errors result.  This extends the memset16
support to EV6.

Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Matt Turner <mattst88@gmail.com>