vo-aacenc: use the BR2_ARM_CPU_ARM* options

The vo-aacenc package has an option to enable ARMv5 optimizations. It
was enabled based on per ARM core options, but now that we have per
ARM architecture options, this commit switches to using them.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>