[libcxx] Fix include directory order

It's important that the arch directory be included first so that
its header files which interpose on the default include dir
be included instead of the default ones. The clang driver [1] does
this when not building with -nostdinc, the libcxx build should
do the same.

We found this after https://reviews.llvm.org/D154282 when cross
compiling from non Linux to Linux. If the host machine was not
Linux, _LIBCPP_HAS_NO_TIME_ZONE_DATABASE would be defined in
the default include dir __config_site, while it was undefined
in the arch specific one causing build failures.

Author: abrachet

libcxx/include/CMakeLists.txt

@@ -1029,8 +1029,10 @@ add_custom_target(generate-cxx-headers ALL DEPENDS ${_all_includes})
 add_library(cxx-headers INTERFACE)
 target_link_libraries(cxx-headers INTERFACE libcxx-abi-headers)
 add_dependencies(cxx-headers generate-cxx-headers)
-target_include_directories(cxx-headers INTERFACE ${LIBCXX_GENERATED_INCLUDE_DIR}
-                                                 ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR})
+# It's important that the arch directory be included first so that its header files
+# which interpose on the default include dir be included instead of the default ones.
+target_include_directories(cxx-headers INTERFACE ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}
+                                                 ${LIBCXX_GENERATED_INCLUDE_DIR})
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})

libcxx/modules/CMakeLists.txt

@@ -128,8 +128,10 @@ if ("${LIBCXX_GENERATED_INCLUDE_DIR}" STREQUAL "${LIBCXX_GENERATED_INCLUDE_TARGE
   # This typically happens when the target is not installed.
   set(LIBCXX_CONFIGURED_INCLUDE_DIRS "${LIBCXX_GENERATED_INCLUDE_DIR}")
 else()
+  # It's important that the arch directory be included first so that its header files
+  # which interpose on the default include dir be included instead of the default ones.
   set(LIBCXX_CONFIGURED_INCLUDE_DIRS
-    "${LIBCXX_GENERATED_INCLUDE_DIR};${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}"
+    "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR};${LIBCXX_GENERATED_INCLUDE_DIR}"
   )
 endif()
 configure_file(