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