[Ada] Remove name constant used by GNATprove but not by GNAT

[Pierre-Marie de Rodat via Gcc-patches]

Revert recent removal of Name_Rpc, which is needed by GNATprove to
detect calls to potentially blocking subprograms.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * snames.ads-tmpl: Restore Name_Rpc.
diff --git a/gcc/ada/snames.ads-tmpl b/gcc/ada/snames.ads-tmpl
--- a/gcc/ada/snames.ads-tmpl
+++ b/gcc/ada/snames.ads-tmpl
@@ -336,6 +336,7 @@ package Snames is
    Name_uCall                          : constant Name_Id := N + $;
    Name_RCI_Name                       : constant Name_Id := N + $;
    Name_Receiver                       : constant Name_Id := N + $;
+   Name_Rpc                            : constant Name_Id := N + $;
    Name_Subp_Id                        : constant Name_Id := N + $;
    Name_Operation                      : constant Name_Id := N + $;
    Name_Argument                       : constant Name_Id := N + $;