ada: Factor out duplicated code in bodies of System.Task_Primitives.Operations

ada: Factor out duplicated code in bodies of System.Task_Primitives.Operations

The duplication is present in some POSIX-like implementations (POSIX
and RTEMS) while it has already been eliminated in others (Linux, QNX).  The
latter implementations are also slightly modified for consistency's sake.

No functional changes.

gcc/ada/

	* libgnarl/s-taprop__dummy.adb (Initialize_Lock): Fix formatting.
	* libgnarl/s-taprop__linux.adb (RTS_Lock_Ptr): Delete.
	(Init_Mutex): Rename into...
	(Initialize_Lock): ...this.
	(Initialize_Lock [Lock]): Call above procedure.
	(Initialize_Lock [RTS_Lock]): Likewise.
	(Initialize_TCB): Likewise.
	* libgnarl/s-taprop__posix.adb (Initialize_Lock): New procedure
	factored out from the other two homonyms.
	(Initialize_Lock [Lock]): Call above procedure.
	(Initialize_Lock [RTS_Lock]): Likewise.
	* libgnarl/s-taprop__qnx.adb (RTS_Lock_Ptr): Delete.
	(Init_Mutex): Rename into...
	(Initialize_Lock): ...this.
	(Initialize_Lock [Lock]): Call above procedure.
	(Initialize_Lock [RTS_Lock]): Likewise.
	(Initialize_TCB): Likewise.
	* libgnarl/s-taprop__rtems.adb (Initialize_Lock): New procedure
	factored out from the other two homonyms.
	(Initialize_Lock [Lock]): Call above procedure.
	(Initialize_Lock [RTS_Lock]): Likewise.