[Ada] Fix retrieval of number of CPUs on QNX

[Ada] Fix retrieval of number of CPUs on QNX

Although the sysconf SC_NPROCESSORS_ONLN is also defined by the API, the
only documented way to retrieve the number of CPUs is by using the syspage.

This also better organise the QNX-specific macros in adaint.c

2018-05-22  Jerome Lambourg  <lambourg@adacore.com>

gcc/ada/

	* adaint.c: Reorganize QNX-specific macros, use syspage to retreive the
	number of CPUs.

From-SVN: r260531