Better error when calling an undefined alien function on x86-64.

Better error when calling an undefined alien function on x86-64.

(sb-alien:alien-funcall
 (sb-alien:extern-alien "nnnn" (function sb-alien:void)))
now says "The alien function "nnnn" is undefined." instead of
"Attempt to call an undefined alien function."

This is achieved by storing the address of the linkage table entry in
RBX before the call. Should be trivial to port to other platforms.