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/iopreds.c

@@ -3284,7 +3284,7 @@ p_peek_mem_write_stream (void)
     if (H + 1024 >= ASP) {
       UNLOCK(Stream[sno].streamlock);
       H = HI;
-      if (!Yap_gcl((ASP-HI)*sizeof(CELL), 3, ENV, P)) {
+      if (!Yap_gcl((ASP-HI)*sizeof(CELL), 3, ENV, gc_P(P,CP))) {
 	UNLOCK(Stream[sno].streamlock);
 	Yap_Error(OUT_OF_STACK_ERROR, TermNil, Yap_ErrorMessage);
 	return(FALSE);
@@ -5842,7 +5842,7 @@ p_same_file(void) {
   {
     struct stat *b1, *b2;
     while ((char *)H+sizeof(struct stat)*2 > (char *)(ASP-1024)) {
-      if (!Yap_gcl(2*sizeof(struct stat), 2, ENV, P)) {
+      if (!Yap_gcl(2*sizeof(struct stat), 2, ENV, gc_P(P,CP))) {
 	Yap_Error(OUT_OF_STACK_ERROR, TermNil, Yap_ErrorMessage);
 	return FALSE;
       }