util: Add a --no-test-xml option to the m5 util's scons.

This forces the test XML output files to be omitted from the build. This
lets you run scons build/ to build everything without running any tests,
and can be used to verify that everything builds correctly even if the
tests don't run/pass.

Change-Id: I280ffe4e76b2249912f9a51a414f4058a3643229
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27755
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>

util/m5/README.md

@@ -175,6 +175,7 @@ some other prefix corresponding to that host.
 ### SCons command line flags
 
 --debug-build: Compile with the -g option, and -O0.
+--no-test-xml: Exclude the test result XML files from the build.
 
 ## External dependency detection