Remove uses of SIGILL for CPU extension detection, that method is not acceptable
in a library.
Should not change anything for PPC, the autodetection is currently pointless due
to other code being compiled with -maltivec as well (and detection for OSX and
AmigaOS remains in place).
SPARC binaries built with VIS support can now only run on systems with VIS.
Originally committed as revision 10648 to svn://svn.ffmpeg.org/ffmpeg/trunk