Use skip_struct for __mcontext because skip_field doesn't seem to work

Author: joshtriplett

libc-test/build.rs

@@ -2935,6 +2935,10 @@ fn test_freebsd(target: &str) {
             // Those are introduced in FreeBSD 15.
             "xktls_session_onedir" | "xktls_session" if Some(15) > freebsd_ver => true,
 
+            // __mcontext changed mc_spare and mc_tlsbase between OS releases, and `skip_field`
+            // doesn't seem to work for them.
+            "__mcontext" => true,
+
             _ => false,
         }
     });
@@ -3074,11 +3078,6 @@ fn test_freebsd(target: &str) {
             // `tcp_snd_wscale` and `tcp_rcv_wscale` are bitfields
             ("tcp_info", "tcp_snd_wscale") => true,
             ("tcp_info", "tcp_rcv_wscale") => true,
-            // mc_spare can change in size between OS releases.  It's a spare field, after all.
-            ("__mcontext", "mc_spare") => true,
-
-            // mc_tlsbase introduced in FreeBSD 15
-            ("__mcontext", "mc_tlsbase") => true,
 
             _ => false,
         }