LIBCXXABI_TARGET_TRIPLE won't always be set.

Fixes issue with r226235. Build configuration difference between
libc++ and libc++abi.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@226240 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed