scons,misc: Update default X86 protocol to MESI_Two_Level

MI_Example is a particularly poor protocol as the default for X86.
MESI_Two_Level is a suitable replacement.

The primary reason for this change is so the vanilla X86 build can run
the X86DemoBoard, submitted here:
https://gem5-review.googlesource.com/c/public/gem5/+/53004

Change-Id: I46212f795684bd1f2ce285c69ffcad2f148ab328
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/53503
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>

tests/gem5/gem5_library_example_tests/test_gem5_library_examples.py

@@ -68,7 +68,6 @@ if os.access("/dev/kvm", mode=os.R_OK | os.W_OK):
         ),
         config_args=[],
         valid_isas=(constants.x86_tag,),
-        protocol="MESI_Two_Level",
         valid_hosts=constants.supported_hosts,
         length=constants.long_tag,
     )