rename --tune to --cpu and make --cpu pass the apropriate -march=xx option to the compiler.

Note that previous implementation (--tune) wasn't consistent with regards to setting -march/-mcpu/-mtune whereas current --cpu now is
Original thread:
Date: Oct 17, 2006 3:20 PM (patch) || Date: Oct 13, 2006 12:03 PM (suggestion of configure clean-up)
Subject: Re: [Ffmpeg-devel] [PATH] test if cpu supports CMOV

Originally committed as revision 6729 to svn://svn.ffmpeg.org/ffmpeg/trunk