* Makefile.rtl: Add s-multip.
* adaint.c: New function __gnat_number_of_cpus, implemented for Linux,
defaulting to 1 for other platforms.
* adaint.h: New function __gnat_number_of_cpus.
* impunit.adb (Non_Imp_File_Names_12): New file list for Ada 2012,
with a single component "s-multip".
* impunit.ads (Kind_Of_Unit): New enumerated value Ada_12_Unit for Ada
2012.
* rtsfind.ads (RTU_Id): New enumerated value System_Multiprocessors
* s-multip.ads, s-multip.adb: New Ada 2012 package.
* sem_ch10.adb (Analyze_With_Clause): Check also Ada 2012 units.