cmdutils: Fix cpuflags so the flags dont pull in and out other flags.

Note, this doesnt break compatibility with libav, as libav
has implemented a incompatible and more limited system under the same
-cpuflags command line option we used since some time.

The differences to libav for example are we can do things like
ffmpeg -cpuflags -sse+mmx -cpuflags +3dnow
Its also possible in our system to force flags that have not been
detected as available
And our -cpuflags works with all tools not just 1

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>