Fix Emscripten's locale

Before, the code was like
https://github.com/emscripten-core/emscripten/blob/dc1abd514b1bade135a01a4453a9ff6def0793b6/system/lib/libcxx/include/__locale_dir/locale_base_api.h#L12-L30

But now they are divided into two parts, one using the new API and the
other using old. See "Locale API reimplementation" above.
https://github.com/llvm/llvm-project/blob/ec28b8f9cc7f2ac187d8a617a6d08d5e56f9120e/libcxx/include/__locale_dir/locale_base_api.h#L116-L138

This adds Emscripten in the beginning of the "old" API list. (This has
to be the beginning; see #23414)

Author: aheejin

system/lib/libcxx/include/__locale_dir/locale_base_api.h

@@ -127,7 +127,9 @@
 //       (by providing global non-reserved names) and the new API. As we move individual platforms
 //       towards the new way of defining the locale base API, this should disappear since each platform
 //       will define those directly.
-#    if defined(_AIX) || defined(__MVS__)
+#    if defined(__EMSCRIPTEN__)
+#      include <xlocale.h>
+#    elif defined(_AIX) || defined(__MVS__)
 #      include <__locale_dir/locale_base_api/ibm.h>
 #    elif defined(__ANDROID__)
 #      include <__locale_dir/locale_base_api/android.h>