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

configure

@@ -2163,7 +2163,7 @@ elif enabled x86; then
 EOF
 
     # check whether EBX is available on x86
-    check_asm ebx_available '""::"b"(0)'
+    check_asm ebx_available '""::"b"(0)' && check_asm ebx_available '"":::"%ebx"'
 
     # check whether more than 10 operands are supported
     check_cc <<EOF && enable ten_operands