ada: Fix race condition in protected entry call

This patch only affects the single-entry implementation of protected
objects.

Before this patch, there was a race condition where a task that
called an entry could put itself to sleep right after another task
had executed the entry as a proxy and signalled the not-yet-waiting
first task, which caused the first task to enter a deadlock.

Note that this race condition has been identified and fixed before
for the implementations of the run-time that live under hie/.

This patch reworks the locking sequence so that it is closer to the
one that's used in the multiple-entry implementation of protected
objects. The code for the multiple-entry implementation is spread
across multiple subprograms. To draw a parallel with the section
this patch modifies, one can read the following subprograms:

- System.Tasking.Protected_Objects.Operations.Protected_Entry_Call
- System.Tasking.Entry_Calls.Wait_For_Completion
- System.Tasking.Entry_Calls.Check_Pending_Actions_For_Entry_Call

This patch also adds a comment that explicitly states the locking
constraint that must hold in the affected section.

gcc/ada/

	* libgnarl/s-tposen.adb: Fix race condition. Add comment to justify
	the locking timing.