package/gst1-libav: fix powerpc build

AS "hack" is also needed for powerpc builds otherwise build fails on:
AS	libavcodec/ppc/fft_altivec.o
/home/buildroot/autobuild/run/instance-2/output/host/bin/powerpc64-linux-as: invalid option -- '_'

This error is raised because CPPFLAGS are (wrongly) passed to AS

Fixes:
 - http://autobuild.buildroot.org/results/6710e72452fe70788d75191b8b735f90beff9361
 - http://autobuild.buildroot.org/results/a1f22975d4a4a6cfc553a450da464a431702800f
 - http://autobuild.buildroot.org/results/be2d161443c5ab24979e6b9389eb21a53d4b4066



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