[Ada] Runtime transition: System.Threads

[Ada] Runtime transition: System.Threads

gcc/ada/

	* libgnat/s-thread.ads: Fix comments.  Remove unused package
	imports.
	(Thread_Body_Exception_Exit): Remove Exception_Occurrence
	parameter.
	(ATSD): Declare type locally.
	* libgnat/s-thread__ae653.adb: Fix comments.  Remove unused
	package imports.  Remove package references to Stack_Limit
	checking.
	(Install_Handler): Remove.
	(Set_Sec_Stack): Likewise.
	(Thread_Body_Enter): Remove calls to Install_Handler and
	Stack_Limit checking.
	(Thread_Body_Exception_Exit): Remove Exception_Occurrence
	parameter.
	(Init_RTS): Call local Get_Sec_Stack.  Remove call to
	Install_Handler.  Remove references to accessors for
	Get_Sec_Stack and Set_Sec_Stack.  Remove OS check.
	(Set_Sec_Stack): Remove.