tests, scons: Fix Testlib test failures

This commit changes the fs/linux/arm and learning_gem5 tests as
they were previously failing with the Ruby change. The
fs/linux/arm long tests require the addition of a new gem5 build,
ARM_X86, which builds the ARM and X86 ISAs with the
MESI_Two_Level cache hierarchy.

ext/testlib/configuration.py

@@ -255,6 +255,9 @@ def define_constants(constants):
     constants.all_compiled_tag = "ALL"
     constants.null_all_ruby = "NULL_All_Ruby"
 
+    # Need this build in order to run the fs/linux/arm testlib tests.
+    constants.arm_x86_tag = "ARM_X86"
+
     constants.variant_tag_type = "variant"
     constants.opt_tag = "opt"
     constants.debug_tag = "debug"