Enable support for atomic primitives on SPARC/Linux

The SPARC/Linux port is very similar to the SPARC/Solaris port nowadays so
it makes sense to copy the setting of the support for atomic primitives.

This fixes the single regression in the gnat.dg testsuite:
FAIL: gnat.dg/prot7.adb (test for excess errors)

gcc/ada/
	* libgnat/system-linux-sparc.ads (Support_Atomic_Primitives): New
	constant set to True.