gst-ffmpeg: work-around bogus configure logic on SPARC

gst-ffmpeg: work-around bogus configure logic on SPARC

The libav version built into the gst-ffmpeg code produces a bogus
binary on SPARC, which causes the following error of the
check-bin-arch script:

  ERROR: architecture for ./usr/lib/gstreamer-0.10/libgstffmpeg.so is Sparc v8+, should be Sparc
  ERROR: architecture for ./usr/lib/gstreamer-0.10/libgstpostproc.so is Sparc v8+, should be Sparc
  ERROR: architecture for ./usr/lib/gstreamer-0.10/libgstffmpegscale.so is Sparc v8+, should be Sparc

The problem is the following bit of code in
gst-lib/ext/libav/configure:

elif enabled sparc; then

    enabled vis && check_asm vis '"pdist %f0, %f0, %f0"' -mcpu=ultrasparc &&
        add_cflags -mcpu=ultrasparc -mtune=ultrasparc

I.e, it checks if the architecture supports the pdist
instruction... but forces -mcpu to ultrasparc while doing so. So it's
like "let's see if this Ultrasparc instruction exists when I force the
compiler to think I'm using Ultrasparc", which is non-sensical. This
has been fixed later on in libav upstream:

  https://git.libav.org/?p=libav.git;a=commit;h=6aa93689abe8c095cec9fa828c2dee3131008995

However, this commit cannot be backported as-is since the shell
function check_inline_asm did not exist in the old libav version
bundled in gst-ffmpeg.

Therefore, we take the simpler route of disabling the VIS
optimizations on SPARCv8 and Leon3.

Fixes:

  http://autobuild.buildroot.net/results/e82d179c3d4f92ad7423693a4b1d42379a3f5411/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>