update isabelle settings to set L4V_ARCH to ARM by default

misc/etc/settings

@@ -26,6 +26,9 @@ JEDIT_JAVA_OPTIONS="-Xms128m -Xmx4096m -Xss4m"
 # show brackets in jEdit
 ISABELLE_JEDIT_OPTIONS="-m brackets"
 
+# Everyone most likely wants ARM
+: ${L4V_ARCH:="ARM"}
+
 # Heap input locations. We have a unique set of heap files for each copy of
 # Isabelle the user has checked out, so that users can have multiple sessions
 # running at the same time without too many problems.