ada: Improve robustness of stack usage tracking in concurrent contexts

ada: Improve robustness of stack usage tracking in concurrent contexts

Enabled computation of stack usage for tasks that have already
initialized their stacks with the expected fill pattern.

Ensured that stack usage snapshots for tasks are taken while
the runtime is locked, to maintain consistency.

For the environment task, approximated the stack origin using
the topmost stack known address during initialization, and take
into account that the Stack_Analyzer object is not part of its
ATCB.

gcc/ada/ChangeLog:

	* libgnarl/s-stusta.adb (Report_Impl): Export a copy of
	the current stack usage while holding the runtime lock.
	(Report_For_Task): Do not compute stack usage for a task
	that has not yet initialized its stack with the expected
	pattern.
	(Report_For_Task): The Stack_Analyzer object for the
	environment task is not part of its ATCB. For the rest of
	the tasks wait until we have initialized the stack pattern
	before computing stack usage.
	(Report_All_Tasks, Get_All_Tasks_Usage,
	Get_Current_Task_Usage): Adapt to the new interface from
	Report_Impl. Take into account that Result_Array can be
	null. When we don't store stack results for a task we
	need to compute it when requested.
	(Print): Handle the case when we don't know the stack
	usage to be reported.
	* libgnat/s-stausa.adb (Initialize): For the environment
	task, approximate the stack origin with the topmost
	stack address that is known.
	* libgnat/s-stausa.ads: Clarify comments.