scons: Only warn about not finding fast model libs if it's enabled.

These warnings could be confusing and aren't useful if it's not.

Change-Id: Ie660f639a3d8ee3406153ceb771b1ba5d0df9225
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58355
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/fastmodel/SConscript

@@ -105,7 +105,8 @@ def staticify(env, name):
         full_name = Dir(path).File(static_name).get_abspath()
         if os.path.isfile(full_name):
             return File(full_name)
-    warning("Failed to find FM static lib: " + name)
+    if env['CONF']['USE_ARM_FASTMODEL']:
+        warning("Failed to find FM static lib: " + name)
     return name
 
 # Adjust the build environment to support building in Fast Models.