ada: Fix System.Interrupt_Names generation on VxWorks

The spec of Ada.Interrupts.Names for VxWorks (and RTEMS) contains a
subtype declaration. This is a deviation from the Ada reference manual
and the sed script used to generate System.Interrupt_Names failed to
handle it. This patch fixes this.

gcc/ada/ChangeLog:

	* xsintnam.sed: Handle special case.