runtime: delete export_arm_test.go

The only thing export_arm_test.go does is to export usplit,
    which does not exist in gccgo.
    
    Reviewed-on: https://go-review.googlesource.com/c/155760

From-SVN: r267435

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-5a9ff61d72c95a50cbbfb0e1bf57646197910363
+416baf55e4890acab244470f6457372987a17a68
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/export_arm_test.go

@@ -1,9 +0,0 @@
-// Copyright 2015 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Export guts for testing.
-
-package runtime
-
-var Usplit = usplit