[Ada] Support both concurrent and sequential partition elaboration policies

[Arnaud Charlet <charlet at adacore dot com>]

In the restricted profile, both policies are now supported. The default one
is concurrent but the sequential can be selected too.

The following should now compile:

pragma Profile (Ravenscar);
pragma Partition_Elaboration_Policy (Concurrent);

package p is
  task t;
end;

package body p is
  task body t is
  begin
    loop
      null;
    end loop;
  end;
end; 

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

2012-11-06  Tristan Gingold  <gingold@adacore.com>

	* exp_ch9.adb (Build_Activation_Chain_Entity): Return immediately if
	partition elaboration policy is sequential.
	(Build_Task_Activation_Call): Likewise. Use
	Activate_Restricted_Tasks on restricted profile.
	(Make_Task_Create_Call): Do not use the _Chain
	parameter if elaboration policy is sequential. Call
	Create_Restricted_Task_Sequential in that case.
	* exp_ch3.adb (Build_Initialization_Call): Change condition to
	support concurrent elaboration policy.
	(Build_Record_Init_Proc): Likewise.
	(Init_Formals): Likewise.
	* bindgen.adb (Gen_Adainit): Declare Partition_Elaboration_Policy
	and set it in generated code if the elaboration policy is
	sequential. The procedure called to activate all tasks is now
	named __gnat_activate_all_tasks.
	* rtsfind.adb (RE_Activate_Restricted_Task,
	RE_Create_Restricted_Task_Sequential): New RE_Id literals.
	* s-tarest.adb (Create_Restricted_Task): Added to create a task without
	adding it on an activation chain.
	(Activate_Tasks): Has now a Chain parameter.
	(Activate_All_Tasks_Sequential): Added. Called by the binder to
	activate all tasks.
	(Activate_Restricted_Tasks): Added. Called during elaboration to
	activate tasks of the units.
	* s-tarest.ads: Remove pragma Partition_Elaboration_Policy.
	(Partition_Elaboration_Policy): New variable (set by the binder).
	(Create_Restricted_Task): Revert removal of the chain parameter.
	(Create_Restricted_Task_Sequential): New procedure.
	(Activate_Restricted_Tasks): Revert removal.
	(Activate_All_Tasks_Sequential): New procedure.

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