Move libc++ settings to __config_site. NFC (#16992)

Author: sbc100

system/lib/libcxx/include/__config

@@ -1,6 +1,3 @@
-// XXX EMSCRIPTEN: macros that would ordinarily be added from __config_site.in
-#define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
-
 // -*- C++ -*-
 //===--------------------------- __config ---------------------------------===//
 //

system/lib/libcxx/include/__config_site

@@ -1,3 +1,4 @@
 // Set the LIBCPP ABI version 2 under emscripten so that we get nicely aligned string
 // data and other nice fixes.
 #define _LIBCPP_ABI_VERSION 2
+#define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS