[libunwind] Fix building standalone after c48974ffd7

After this change, we need to explicitly list the languages the
project uses, otherwise the assembly source files won't get built
at all.

Previously (before that commit), the assembly source files were
simply treated as C.

The toplevel llvm CMakeLists.txt adds these three languages, so
when building libunwind integrated as part of that, it works fine.

libunwind/CMakeLists.txt

@@ -16,7 +16,7 @@ set(CMAKE_MODULE_PATH
   )
 
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBUNWIND_STANDALONE_BUILD)
-  project(libunwind)
+  project(libunwind LANGUAGES C CXX ASM)
 
   # Rely on llvm-config.
   set(CONFIG_OUTPUT)