sim: testsuite: push $arch out to targets

This is needed to move to automake & its dejagnu-provided logic,
and eventually by the unified sim logic.  The $arch is used only
to figure out which `run` program to use when running tests, and
as we move to a single top-level build, we can delete this and
use sim/run directly.

sim/testsuite/arm/xscale/xscale.exp

@@ -1,8 +1,9 @@
 # XSCALE simulator testsuite.
 
 if { [istarget arm*-*-*] } {
-    # load support procs (none yet)
-    # load_lib cgen.exp
+    # Used to locate the `run` program.
+    global arch
+    set arch "arm"
 
     # all machines
     set all_machs "xscale"