[Ada] RTEMS: use hardware interrupts instead of signals for interrupt handling

[Pierre-Marie de Rodat <derodat@adacore.com>]

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RTEMS supports attaching interrupt handlers to hardware interrupt
vectors, which is superior to the current approach of attaching handlers
to signals.  Direct attachment of handlers removes the execution
overhead of converting hardware interrupts to signals and their
subsequent propagation to the interrupt manager. It also removes the
limitation of the number of hardware interrupts that can be supported
under the signals model, as RTEMS is limited to 32 signals.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* Makefile.rtl (VxWorks): Rename s-inmaop__vxworks.adb to
	s-inmaop__hwint.adb.
	(RTEMS): Use s-inmaop__hwint.adb, s-intman__rtems.adb/s,
	s-taprop__rtems.adb.
	* libgnarl/a-intnam__rtems.ads: Remove signals definitions and
	replace with Hardware_Interrupts.
	* libgnarl/s-inmaop__vxworks.adb: Rename as...
	* libgnarl/s-inmaop__hwint.adb: ... this.
	* libgnarl/s-interr__hwint.adb: Remove unnecessary comments.
	* libgnarl/s-intman__rtems.ads, libgnarl/s-intman__rtems.adb:
	New files.
	* libgnarl/s-osinte__rtems.adb: Add RTEMS API bindings.
	(Binary_Semaphore_Create, Binary_Semaphore_Delete,
	Binary_Semaphore_Obtain, Binary_Semaphore_Release,
	Binary_Semaphore_Flush, Interrupt_Connect,
	Interrupt_Number_To_Vector): New functions.
	* libgnarl/s-osinte__rtems.ads (Num_HW_Interrupts, Signal):
	Removed.
	(NSIG, Interrupt_Range): New.
	(Binary_Semaphore_Create, Binary_Semaphore_Delete,
	Binary_Semaphore_Obtain, Binary_Semaphore_Release,
	Binary_Semaphore_Flush, Interrupt_Connect,
	Interrupt_Number_To_Vector): Remove Import pragma.
	* libgnarl/s-taprop__rtems.adb: New file.

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