Fix ARM_CC_FOR_TARGET handling

The previous patch that introduced the arm_cc_for_target procedure
moved the ARM_CC_FOR_TARGET global check to that procedure, but forgot
to tell tcl that ARM_CC_FOR_TARGET is a global.  As a result,
specifying ARM_CC_FOR_TARGET on the command line actually does
nothing.  This fixes it.

Change-Id: I4e33b7633fa665e2f7b8f8c9592a949d74a19153
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 67e5838..37accd9 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -8671,12 +8671,12 @@
 # that the executable they produce can run.
 
 gdb_caching_proc arm_cc_for_target {
-    if {[info exists ARM_CC_FOR_TARGET]} {
+    if {[info exists ::ARM_CC_FOR_TARGET]} {
 	# If the user specified the compiler explicitly, then don't
 	# check whether the resulting binary runs outside GDB.  Assume
 	# that it does, and if it turns out it doesn't, then the user
 	# should get loud FAILs, instead of UNSUPPORTED.
-	return $ARM_CC_FOR_TARGET
+	return $::ARM_CC_FOR_TARGET
     }
 
     # Fallback to a few common compiler names.  Also confirm the