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>

package/gstreamer1/gst1-libav/gst1-libav.mk

@@ -15,8 +15,8 @@ GST1_LIBAV_LICENSE = GPL-2.0+
 GST1_LIBAV_LICENSE_FILES = COPYING
 GST1_LIBAV_CONF_EXTRA_OPTS = --cross-prefix=$(TARGET_CROSS) --target-os=linux
 
-# fixes arm build: https://bugzilla.gnome.org/show_bug.cgi?id=694416
-ifeq ($(BR2_arm)$(BR2_armeb)$(BR2_aarch64)$(BR2_aarch64_be),y)
+# fixes arm/powerpc build: https://bugzilla.gnome.org/show_bug.cgi?id=694416
+ifeq ($(BR2_arm)$(BR2_armeb)$(BR2_aarch64)$(BR2_aarch64_be)$(BR2_powerpc)$(BR2_powerpc64)$(BR2_powerpc64le),y)
 GST1_LIBAV_CONF_ENV = AS="$(TARGET_CROSS)gcc"
 endif