x86: Tighten register constraints for decode_significance*_x86.

On 32-bit OS X with gcc 4.0/4.2 and shared libraries enabled, the ebx register
is not available, but required to assemble the functions.

This reverts commit 8742a4ff to a simplified version of the original constraints.

libavcodec/h264_cabac.c

@@ -1656,7 +1656,7 @@ decode_cabac_residual_internal(H264Context *h, DCTELEM *block,
             index[coeff_count++] = last;\
         }
         const uint8_t *sig_off = significant_coeff_flag_offset_8x8[MB_FIELD];
-#if ARCH_X86 && HAVE_6REGS && !defined(BROKEN_RELOCATIONS)
+#if ARCH_X86 && HAVE_EBX_AVAILABLE && !defined(BROKEN_RELOCATIONS)
         coeff_count= decode_significance_8x8_x86(CC, significant_coeff_ctx_base, index,
                                                  last_coeff_ctx_base, sig_off);
     } else {

libavcodec/x86/h264_i386.h

@@ -36,7 +36,7 @@
 
 //FIXME use some macros to avoid duplicating get_cabac (cannot be done yet
 //as that would make optimization work hard)
-#if HAVE_6REGS && !defined(BROKEN_RELOCATIONS)
+#if HAVE_EBX_AVAILABLE && !defined(BROKEN_RELOCATIONS)
 static int decode_significance_x86(CABACContext *c, int max_coeff,
                                    uint8_t *significant_coeff_ctx_base,
                                    int *index, x86_reg last_off){
@@ -144,6 +144,6 @@ static int decode_significance_8x8_x86(CABACContext *c,
     );
     return coeff_count;
 }
-#endif /* HAVE_6REGS && !defined(BROKEN_RELOCATIONS) */
+#endif /* HAVE_EBX_AVAILABLE && !defined(BROKEN_RELOCATIONS) */
 
 #endif /* AVCODEC_X86_H264_I386_H */