arch-arm,base: Use SourceLib() in a few simple spots.

There are a couple places where SourceLib(), an interface to SCons's
LIB variable that respects tags, can be used simply in existing
SConscripts. Do that so that SourceLib is used, and as an example if
someone wants to see it in action.

Change-Id: Idb27da17724990093252b710f5f9fde8351a4bc5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58070
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/SConscript

@@ -48,7 +48,7 @@ else:
     warning("No IEEE FP rounding mode control.\n"
             "FP results may deviate slightly from other platforms.")
 if env['HAVE_PNG']:
-    env.Append(LIBS=['png'])
+    SourceLib('png')
     Source('pngwriter.cc')
 Source('fiber.cc')
 GTest('fiber.test', 'fiber.test.cc', 'fiber.cc')