fix cmsg test proposal

Author: devnexen

libc-test/test/cmsg.rs

@@ -11,6 +11,8 @@ mod t {
 
     extern "C" {
         pub fn cmsg_firsthdr(msgh: *const msghdr) -> *mut cmsghdr;
+        // see below
+        #[cfg(not(target_arch = "sparc64"))]
         pub fn cmsg_nxthdr(mhdr: *const msghdr, cmsg: *const cmsghdr) -> *mut cmsghdr;
         pub fn cmsg_space(length: c_uint) -> usize;
         pub fn cmsg_len(length: c_uint) -> usize;