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/ldst-unsignedimm.ll b/llvm/test/CodeGen/AArch64/ldst-unsignedimm.ll
index aa513f5..1e7540d 100644
--- a/llvm/test/CodeGen/AArch64/ldst-unsignedimm.ll
+++ b/llvm/test/CodeGen/AArch64/ldst-unsignedimm.ll
@@ -1,4 +1,4 @@
-; RUN: llc -verify-machineinstrs < %s -march=aarch64 | FileCheck %s
+; RUN: llc -verify-machineinstrs < %s -mtriple=aarch64-none-linux-gnu | FileCheck %s
 
 @var_8bit = global i8 0
 @var_16bit = global i16 0