gst1-libav: allow usage of ARMv6 optimizations on ARMv7 platforms

Contrary to the ffmpeg package, the gst1-libav package was only
allowing ARMv6 optimizations on ARMv6 platforms. This commit also
allows ARMv6 optimizations on ARMv7 platforms, like the ffmpeg package
does.

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