ART: Fix movnti assembler

Movnti was receiving rex prefix before its opcode. Additionally,
the 64-bit version was missing the rex.w prefix.

Change-Id: Ie5c3bbe109765a0b990cafeeea1ee30329daabd0
Signed-off-by: Razvan A Lupusoru <razvan.a.lupusoru@intel.com>
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>