package/opencv3: fix aarch64 build

Commit a17402e4 ("opencv3:
conditionally enable NEON and VFPv3 optimizations") broke the build on
AArch64, because the VFPv3 optimizations are not supported by OpenCV
on this architecture:

  https://github.com/opencv/opencv/blob/3.4/cmake/OpenCVCompilerOptimizations.cmake#L275

However BR2_ARM_CPU_HAS_VFPV3 is always true on AArch64, causing this
build failure for every AArch64 build of OpenCV 3. This commit fixes
that by enabling VFPV3 only on ARM, and not on AArch64.

Fixes:

  http://autobuild.buildroot.net/results/d38fb092cffe086f93b469d1181da77fba0994bd/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>