package/gst1-libav: fix build on mips64el

Apply the same behavior for CPU in gst1-libav than in ffmpeg:
 - use generic cpu for all mips architectures
 - use GCC_TARGET_{ARCH,CPU} instead of BR2_TARGET_{ARCH,CPU}

Fixes:
 - http://autobuild.buildroot.org/results/776028a43bd180b4895efc74321e3aee56cbcb47



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>