[m42] Add arm64 support to ffmpeg

[m42] Add arm64 support to ffmpeg

The following commands were executed within arm64 GBS wayland chroot:

$ # enter chroot
$ gbs chroot --root ~/GBS-ROOT-3.0-MOBILE-ARM64-WAYLAND/local/BUILD-ROOTS/scratch.aarch64.0/
$ # copy over src/third_party/ffmpeg to chroot (e.g.: /home/abuild)
$ cd ffmpeg
$ ./chromium/scripts/build_ffmpeg.py linux arm64
$ ./chromium/scripts/copy_config.sh
$ # edit chromium/scripts/generate_gyp.py; add "arm64" to support platforms.
$ python chromium/scripts/generate_gyp.py
$ edit ffmpeg.gyp; add "asm_sources" to the build
$ # generate the patch against a unchanged ffmpeg copy

Bug: http://web.sec.samsung.net/bugzilla/show_bug.cgi?id=12490
Reviewed by: SeungSeop Park, arno renevier

Change-Id: I7aaab106225cf9a65d97977e9c24220d7ab863c5
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>