Include new instruction execute_cpred to perform tail optimisation for

builtins. Required changes:
- be careful about creeping in deallocate: it may be followed by
something that is not a cut nor a proceed.
- include new instruction in absmi.c: it is a merge of execute and
call_cpred.
- change compiler to generate execute even for C builtins.
- be careful with dexecute: it may not be done if execute_op is a C
builtin.
- if we are in execute_cpred, the garbage collector cannot trust P:
instead it must look at CP to find out the size of the current
environment. The macro gc_P receives that information.
- We don't need to change CP if we do a meta-call from within
execute_cpred (and we in fact cannot). Check places where we do
meta-calls: exec, clause in cdmgr, and lu_recorded.

C/c_interface.c

@@ -536,7 +536,7 @@ doexpand(UInt sz)
   } else {
     arity = 0;
   }
-  if (!Yap_gcl(sz, arity, ENV, P)) {
+  if (!Yap_gcl(sz, arity, ENV, gc_P(P,CP))) {
     return FALSE;
   }
   return TRUE;