gstreamer: provide necessary config value for AArch64

Provide the as_cv_unaligned_access value for the AArch64 architecture.

Fixes:

  http://autobuild.buildroot.net/results/5d6d76451a00d79e1bdfe87b6ced61807666cc48/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>