Only define x86 register names on x86.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15328 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavutil/x86_cpu.h

@@ -43,7 +43,7 @@ typedef int64_t x86_reg;
 #    define REGd    rdx
 #    define REGSP   rsp
 
-#else
+#elif ARCH_X86_32
 
 #    define REG_a "eax"
 #    define REG_b "ebx"