XFAIL ptrace test on arm

The patch adds better target_triple and target_arch defaults for lit tests,
which allows us to XFAIL tests based on architecture.

Was:
  target_triple = LLVM_DEFAULT_TARGET_TRIPLE
  target_arch = HOST_ARCH

Now:
  target_triple = COMPILER_RT_TEST_TARGET_TRIPLE
                , otherwise LLVM_DEFAULT_TARGET_TRIPLE
  target_arch = first item in COMPILER_RT_TEST_TARGET_TRIPLE

Differential Revision: http://reviews.llvm.org/D3855

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209256 91177308-0d34-0410-b5e6-96231b3b80d8