Add explicit triples to AArch64 tests

Only Linux is supported at the moment, and other platforms quickly fault. As a
result these tests would fail on non-Linux hosts. It may be worth making the
tests more generic again as more platforms are supported.

llvm-svn: 174170
diff --git a/llvm/test/CodeGen/AArch64/atomic-ops.ll b/llvm/test/CodeGen/AArch64/atomic-ops.ll
index 8a1e976..bcb1a6f 100644
--- a/llvm/test/CodeGen/AArch64/atomic-ops.ll
+++ b/llvm/test/CodeGen/AArch64/atomic-ops.ll
@@ -1,4 +1,4 @@
-; RUN: llc -march=aarch64 -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-none-linux-gnu -verify-machineinstrs < %s | FileCheck %s
 
 @var8 = global i8 0
 @var16 = global i16 0