Don't prefix yasm with cross prefix.

Patch by Michael Kostylev <mik at it-1 dot ru>

Originally committed as revision 16463 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1277,7 +1277,6 @@ cc_default="${cross_prefix}${cc_default}"
 nm_default="${cross_prefix}${nm_default}"
 ranlib="${cross_prefix}${ranlib}"
 strip="${cross_prefix}${strip}"
-yasmexe="${cross_prefix}${yasmexe}"
 
 set_default cc nm
 enabled cross_compile || host_cc_default=$cc