ffmpeg: add libatomic for armv5

The GNU Atomic library is a GCC support runtime library for atomic
operations not supported by hardware.

Otherwise we get the following when building:

	| arm-oe-linux-gnueabi-gcc -mcpu=arm926ej-s -mfpu=vfp -mfloat-abi=hard -marm --sysroot=.../build/tmp-glibc/work/armv5ehf-vfp-oe-linux-gnueabi/ffmpeg/4.4-r0/recipe-sysroot -Llibavcodec -Llibavdevice -Llibavfilter -Llibavformat -Llibavresample -Llibavutil -Llibpostproc -Llibswscale -Llibswresample -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed  --sysroot=.../build/tmp-glibc/work/armv5ehf-vfp-oe-linux-gnueabi/ffmpeg/4.4-r0/recipe-sysroot -mcpu=arm926ej-s -Wl,--as-needed -Wl,-z,noexecstack -Wl,--warn-common -Wl,-rpath-link=:libpostproc:libswresample:libswscale:libavfilter:libavdevice:libavformat:libavcodec:libavutil:libavresample   -o ffprobe_g fftools/cmdutils.o fftools/ffprobe.o  -lavdevice -lavfilter -lavformat -lavcodec -lavresample -lswresample -lswscale -lavutil  -lm -lxcb -lxcb-shm -lxcb-shape -lxcb-xfixes -lasound -lXv -lX11 -lXext -pthread -lm -lm -lbz2 -lz -pthread -lm -llzma -lz -ltheoraenc -ltheoradec -logg -lm -lm -lm -pthread -lm -lXv -lX11 -lXext
	| .../build/tmp-glibc/work/armv5ehf-vfp-oe-linux-gnueabi/ffmpeg/4.4-r0/recipe-sysroot-native/usr/bin/arm-oe-linux-gnueabi/../../libexec/arm-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/11.1.1/ld: libavformat/libavformat.so: undefined reference to `__atomic_fetch_sub_8'
	| .../build/tmp-glibc/work/armv5ehf-vfp-oe-linux-gnueabi/ffmpeg/4.4-r0/recipe-sysroot-native/usr/bin/arm-oe-linux-gnueabi/../../libexec/arm-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/11.1.1/ld: libavformat/libavformat.so: undefined reference to `__atomic_store_8'
	| .../build/tmp-glibc/work/armv5ehf-vfp-oe-linux-gnueabi/ffmpeg/4.4-r0/recipe-sysroot-native/usr/bin/arm-oe-linux-gnueabi/../../libexec/arm-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/11.1.1/ld: libavformat/libavformat.so: undefined reference to `__atomic_load_8'
	| .../build/tmp-glibc/work/armv5ehf-vfp-oe-linux-gnueabi/ffmpeg/4.4-r0/recipe-sysroot-native/usr/bin/arm-oe-linux-gnueabi/../../libexec/arm-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/11.1.1/ld: libavformat/libavformat.so: undefined reference to `__atomic_fetch_add_8'
	| collect2: error: ld returned 1 exit status

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>