[SystemZ][z/OS] Make LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL available for external unwind library.

On z/OS, we need to pass the location of unwind interface header when building cxxabi. The cmake macro `LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL` is available for this purpose but it is only used with conjunction with `LIBCXXABI_USE_LLVM_UNWINDER`. For the external unwind library we need to use LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL unconditionally whenever it is set.

Reviewed By: #libc_abi, muiez, phosek, SeanP

Differential Revision: https://reviews.llvm.org/D147460

Author: zibi2

Diff for: libcxxabi/CMakeLists.txt

@@ -459,10 +459,10 @@ if (LIBCXXABI_USE_LLVM_UNWINDER OR LLVM_NATIVE_ARCH MATCHES ARM)
   if (LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL STREQUAL "LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL-NOTFOUND")
     set(LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL "")
   endif()
+endif()
 
-  if (NOT LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL STREQUAL "")
-    include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}")
-  endif()
+if (NOT "${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}" STREQUAL "")
+  include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}")
 endif()
 
 # Add source code. This also contains all of the logic for deciding linker flags