libada: Check for the presence of _SC_NPROCESSORS_ONLN

libada: Check for the presence of _SC_NPROCESSORS_ONLN

Check for the presence of _SC_NPROCESSORS_ONLN rather than using a list
of OS-specific macros to decide whether to use `sysconf' like elsewhere
across GCC sources, fixing a compilation error:

adaint.c: In function '__gnat_number_of_cpus':
adaint.c:2398:26: error: '_SC_NPROCESSORS_ONLN' undeclared (first use in this function)
 2398 |   cores = (int) sysconf (_SC_NPROCESSORS_ONLN);
      |                          ^~~~~~~~~~~~~~~~~~~~
adaint.c:2398:26: note: each undeclared identifier is reported only once for each function it appears in

at least with with VAX/NetBSD 1.6.2.

	gcc/ada/
	* adaint.c (__gnat_number_of_cpus): Check for the presence of
	_SC_NPROCESSORS_ONLN rather than a list of OS-specific macros
	to decide whether to use `sysconf'.