Fix the build with nasm.

Use eax instead of rax.

BUG=aomedia:546

Change-Id: Ie3f704557d428d84531311a6bc690293465a622c

aom_dsp/x86/highbd_subpel_variance_impl_sse2.asm

@@ -75,7 +75,7 @@ SECTION .text
   paddd                m6, m4
   mov                  r1, ssem         ; r1 = unsigned int *sse
   movd               [r1], m7           ; store sse
-  movd                rax, m6           ; store sum as return value
+  movd                eax, m6           ; store sum as return value
 %endif
   RET
 %endmacro