autoconf: check that named registers work

configure.ac

@@ -418,7 +418,13 @@ AS_IF([test "$enable_asm" != "no"],[
 #else
 # error !x86_64
 #endif
-__asm__ __volatile__ ("pxor %xmm12,%xmm6");
+unsigned char i = 0, o = 0, t;
+__asm__ __volatile__ ("pxor %%xmm12, %%xmm6 \n"
+                      "movb (%[i]), %[t] \n"
+                      "addb %[t], (%[o]) \n"
+                      : [t] "=&r"(t)
+                      : [o] "D"(&o), [i] "S"(&i)
+                      : "memory", "flags", "cc");
 ]])],
   [AC_MSG_RESULT(yes)
    AC_DEFINE([HAVE_AMD64_ASM], [1], [x86_64 asm code can be used])