sad4d_neon.c -> sadxd_neon.c in prep. for adding SAD3D code

Rename sad4d_neon.c -> sadxd_neon.c in preparation for adding SAD3D
Neon paths in a subsequent patch.

Change-Id: I6512ed1085a03d8fa9a4360d28e304817044cb30

aom_dsp/aom_dsp.cmake

diff --git a/aom_dsp/aom_dsp.cmake b/aom_dsp/aom_dsp.cmake
index 6e11237..6dae95e 100644
--- a/aom_dsp/aom_dsp.cmake
+++ b/aom_dsp/aom_dsp.cmake
@@ -257,7 +257,7 @@
               "${AOM_ROOT}/aom_dsp/x86/obmc_variance_sse4.c")
 
   list(APPEND AOM_DSP_ENCODER_INTRIN_NEON
-              "${AOM_ROOT}/aom_dsp/arm/sad4d_neon.c"
+              "${AOM_ROOT}/aom_dsp/arm/sadxd_neon.c"
               "${AOM_ROOT}/aom_dsp/arm/sad_neon.c"
               "${AOM_ROOT}/aom_dsp/arm/masked_sad_neon.c"
               "${AOM_ROOT}/aom_dsp/arm/subpel_variance_neon.c"

aom_dsp/arm/sadxd_neon.c

diff --git a/aom_dsp/arm/sad4d_neon.c b/aom_dsp/arm/sadxd_neon.c
similarity index 100%
rename from aom_dsp/arm/sad4d_neon.c
rename to aom_dsp/arm/sadxd_neon.c