Use a "b" constraint instead of ebx clobber to detect if EBX is available.

The test using clobber incorrectly indicates ebx to be available with the
default compilers on OpenBSD 4.5 and Haiku.

Originally committed as revision 20101 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2162,7 +2162,7 @@ elif enabled x86; then
 EOF
 
     # check whether EBX is available on x86
-    check_asm ebx_available '"":::"%ebx"'
+    check_asm ebx_available '""::"b"(0)'
 
     # check whether more than 10 operands are supported
     check_cc <<EOF && enable ten_operands