ada: Sync different variants of interrupt handler registration

ada: Sync different variants of interrupt handler registration

This patch propagates the apparently cleanest solutions between various
variants of the runtime units for interrupt handler registration.

In particular, the unnecessary default expressions for list cells with
interrupt handler addresses are removed, the list is changed from
doubly-linked to singly-linked, and assertion preventing registration of
null addresses now appears in all runtimes.

Effectively, it is just a code cleanup and minor optimization; behavior
of the runtime unit is unaffected.

gcc/ada/

	* libgnarl/s-interr.adb
	(Registered_Handler): Remove default expression.
	(Registered_Handlers): Switch to singly-linked list.
	(Bind_Interrupt_To_Entry): Sync whitespace with other unit variants.
	(Is_Registered): Use singly-linked list.
	(Register_Interrupt_Handler): Use singly-linked list and initialized
	allocator; sync assertion with other unit variants.
	* libgnarl/s-interr__sigaction.adb: Likewise.
	* libgnarl/s-interr__vxworks.adb: Likewise.
	* libgnarl/s-interr__hwint.adb: Likewise.
	(Is_Registered): Remove repeated declaration.