tests: Added GTests for base/atomicio.cc

Change-Id: I586a06c70f4e7331b4a31208ef7831e8473509c5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21699
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/SConscript

@@ -35,6 +35,7 @@ if env['CP_ANNOTATE']:
     Source('cp_annotate.cc')
 SimObject('Graphics.py')
 Source('atomicio.cc')
+GTest('atomicio.test', 'atomicio.test.cc', 'atomicio.cc')
 Source('bitfield.cc')
 Source('imgwriter.cc')
 Source('bmpwriter.cc')