Remove uses of SIGILL for CPU extension detection, that method is not acceptable

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.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@10648 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b