Fix aarch64 compile by adding HAVE_ARMV8 define

Fixes build errors seen on the buildroot autobuilders:
http://autobuild.buildroot.net/results/5f8/5f85c32eb89aac48ae8da892d9800bd13274cd3e/build-end.log

libavutil/aarch64/cpu.c: In function 'ff_get_cpu_flags_aarch64':
libavutil/aarch64/cpu.c:25:32: error: 'HAVE_ARMV8' undeclared (first use in this function)
     return AV_CPU_FLAG_ARMV8 * HAVE_ARMV8 |

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
(patch sent upstream:
 http://lists.mplayerhq.hu/pipermail/mplayer-dev-eng/2016-May/073496.html)

Index: configure
===================================================================
--- a/configure	(revision 37871)
+++ b/configure	(working copy)
@@ -1445,6 +1445,8 @@
   --disable-armv6) _armv6=no ;;
   --enable-armv6t2) _armv6t2=yes ;;
   --disable-armv6t2) _armv6t2=no ;;
+  --enable-armv8) _armv8=yes ;;
+  --disable-armv8) _armv8=no ;;
   --enable-armvfp) _armvfp=yes ;;
   --disable-armvfp) _armvfp=no ;;
   --enable-vfpv3) vfpv3=yes ;;
@@ -3261,7 +3263,7 @@
   echores "$_iwmmxt"
 fi
 
-cpuexts_all='ALTIVEC XOP AVX AVX2 FMA3 FMA4 MMX MMX2 MMXEXT AMD3DNOW AMD3DNOWEXT SSE SSE2 SSE3 SSSE3 SSE4 SSE42 FAST_CMOV I686 FAST_CLZ ARMV5TE ARMV6 ARMV6T2 VFP VFPV3 SETEND NEON IWMMXT MMI VIS MVI'
+cpuexts_all='ALTIVEC XOP AVX AVX2 FMA3 FMA4 MMX MMX2 MMXEXT AMD3DNOW AMD3DNOWEXT SSE SSE2 SSE3 SSSE3 SSE4 SSE42 FAST_CMOV I686 FAST_CLZ ARMV5TE ARMV6 ARMV6T2 ARMV8 VFP VFPV3 SETEND NEON IWMMXT MMI VIS MVI'
 test "$_altivec"   = yes && cpuexts="ALTIVEC $cpuexts"
 test "$_mmx"       = yes && cpuexts="MMX $cpuexts"
 test "$_mmxext"    = yes && cpuexts="MMX2 $cpuexts"
@@ -3285,6 +3287,7 @@
 test "$_armv5te"   = yes && cpuexts="ARMV5TE $cpuexts"
 test "$_armv6"     = yes && cpuexts="ARMV6 $cpuexts"
 test "$_armv6t2"   = yes && cpuexts="ARMV6T2 $cpuexts"
+test "$_armv8"     = yes && cpuexts="ARMV8 $cpuexts"
 test "$_armvfp"    = yes && cpuexts="VFP $cpuexts"
 test "$vfpv3"      = yes && cpuexts="VFPV3 $cpuexts"
 test "$setend"     = yes && cpuexts="SETEND $cpuexts"