[libunwind][CMake] Set TARGET_TRIPLE if LIBUNWND_TARGET_TRIPLE is set

This follows the setup used by other runtimes and is expected by
the lit configuration.

Differential Revision: https://reviews.llvm.org/D40820

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@319830 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed