package/ffmpeg: fix nios2 fenv build failure

Fixes
http://autobuild.buildroot.net/results/216/216fbc33c4838b1414cb25f823fd1db5c92e33c6/
http://autobuild.buildroot.net/results/800/800fe1d79ef2e52b86a97feeba85b5c47aff8dcd/
http://autobuild.buildroot.net/results/8a6/8a6d24ecc6487fd20ba663f71996160124083a68/



Only the affected toolchains are disabled. When using branch next/ compilation
works with a buildroot-compiled toolchain using this defconfig:

BR2_nios2=y
BR2_PACKAGE_FFMPEG=y
BR2_PACKAGE_FFMPEG_GPL=y
BR2_PACKAGE_FFMPEG_NONFREE=y
BR2_PACKAGE_FFMPEG_FFPLAY=y
BR2_PACKAGE_FFMPEG_FFSERVER=y
BR2_PACKAGE_FFMPEG_FFPROBE=y
BR2_PACKAGE_FFMPEG_AVRESAMPLE=y
BR2_PACKAGE_FFMPEG_POSTPROC=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>