Disable C++20 time zone support

LLVM 18.1.2 adds C++20 time zone support
(https://libcxx.llvm.org/DesignDocs/TimeZone.html) to libcxx, which
requires access IANA Time Zone Database. Currently it seems it only
supports Linux:
https://github.com/llvm/llvm-project/blob/26a1d6601d727a96f4301d0d8647b5a42760ae0c/libcxx/src/tz.cpp#L45-L49

So this excludes the two source files from build (which is done via
`CMakeLists.txt` in the upstream LLVM) and sets
`_LIBCPP_HAS_NO_TIME_ZONE_DATABASE` macro in `__config_site`. In future
maybe we can consider implementing this in JS.

system/lib/libcxx/include/__config_site

@@ -8,3 +8,4 @@
 #define _LIBCPP_PSTL_CPU_BACKEND_SERIAL
 // __libcpp_verbose_abort creases code size too much
 #define _LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT
+#define _LIBCPP_HAS_NO_TIME_ZONE_DATABASE

tools/system_libs.py

@@ -1617,6 +1617,10 @@ class libcxx(NoExceptLibrary, MTLibrary):
     'support.cpp',
     'int128_builtins.cpp',
     'libdispatch.cpp',
+    # Emscripten does not have C++20's time zone support which requires access
+    # to IANA Time Zone Database. TODO Implement this using JS timezone
+    'tz.cpp',
+    'tzdb_list.cpp',
   ]
 
   def get_cflags(self):