libav: re-enable neon optimizations for arm/armv7 on android

Submitted by Rahul Chaudhry

Link to original bug (#795243)

Description

Created attachment 370915
patch to ffmpeg for avoiding conditional branch to PLT in THUMB-2

Neon optimizations are disabled for libav (see https://bugzilla.gnome.org/show_bug.cgi?id=793980).

See https://github.com/android-ndk/ndk/issues/337 for background.

The current workaround is to disable neon during gstreamer build.

The commit to disable the optimizations is https://github.com/freedesktop/gstreamer-cerbero/commit/41556c415739fbc3a72c7eaee7e70a565b719b2f

I've sent a patch to upstream ffmpeg that should resolve the linker issue (conditional branch to PLT in THUMB-2).
The patch is here: http://ffmpeg.org/pipermail/ffmpeg-devel/2018-April/228246.html (also attached with this bug).

Once this patch makes it into the copy of ffmpeg used in gstreamer build, neon optimizations can be re-enabled.

Patch 370915, "patch to ffmpeg for avoiding conditional branch to PLT in THUMB-2":
0001-swresample-arm-avoid-conditional-branch-to-PLT-in-TH.patch