Re-add check for compilation failure with ebx clobber, and only set
ebx_available if both "b" constraint and exb clobber code compile.
This is to fix compilation with Mac OSX 10.5.8's gcc 4.0.1 in 64
bit mode (-m64) which only fails for ebx/rbx clobbers but not
"b" constraints - this is probably a compiler bug.

Originally committed as revision 20107 to svn://svn.ffmpeg.org/ffmpeg/trunk
