util: Sort the "updater tags" in cpt_upgrader.py --get-cc-file.

This ensures that the same tags will create the same file, and avoids
spurious rebuilds/relinks for null builds.

Change-Id: Ic8e37a24e2c60d74d8c921dde1c5e102d3a764e3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48360
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

util/cpt_upgrader.py

@@ -297,7 +297,7 @@ if __name__ == '__main__':
         print("{")
         print()
         print("std::set<std::string> version_tags = {")
-        for tag in Upgrader.tag_set:
+        for tag in sorted(Upgrader.tag_set):
             print("  \"{}\",".format(tag))
         print("};")
         print()