ada: Tweak handling of thread ID on POSIX

ada: Tweak handling of thread ID on POSIX

This patch changes the task initialization subprograms on POSIX
platforms so that the thread ID of an ATCB is only set once.
This has the advantage of getting rid of the Atomic aspect on
the corresponding record component, and silences a Helgrind
warning about a data race.

gcc/ada/

	* libgnarl/s-taprop__linux.adb (Enter_Task): Move setting
	of thread ID out of Enter_Task.
	(Initialize): Set thread ID for the environment task.
	(Create_Task): Remove now unnecessary Unrestricted_Access
	attribute and add justification for a memory write.
	* libgnarl/s-taprop__posix.adb: Likewise.
	* libgnarl/s-taprop__qnx.adb: Likewise.
	* libgnarl/s-taprop__rtems.adb: Likewise.
	* libgnarl/s-taprop__solaris.adb: Likewise.
	* libgnarl/s-taspri__posix.ads: Remove pragma Atomic for
	Private_Data.Thread, and update documentation comment.
	* libgnarl/s-taspri__lynxos.ads: Likewise.
	* libgnarl/s-taspri__posix-noaltstack.ads: Likewise.
	* libgnarl/s-taspri__solaris.ads: Likewise.
	* libgnarl/s-tporft.adb (Register_Foreign_Thread): Adapt to
	Enter_Task not setting the thread ID anymore.
	* libgnarl/s-tassta.adb (Task_Wrapper): Update comment.