Use -unwindlib=libunwind instead of merging libunwind into libc++

This makes the build of libcxx a bit cleaner, and simplifies later
building and testing of libcxx.

wrappers/clang-target-wrapper.c

@@ -224,6 +224,7 @@ int _tmain(int argc, TCHAR* argv[]) {
     exec_argv[arg++] = _T("-target");
     exec_argv[arg++] = target;
     exec_argv[arg++] = _T("-rtlib=compiler-rt");
+    exec_argv[arg++] = _T("-unwindlib=libunwind");
     exec_argv[arg++] = _T("-stdlib=libc++");
     exec_argv[arg++] = _T("-fuse-ld=lld");
     exec_argv[arg++] = _T("-Qunused-arguments");