Provide a value for ELAST.

Author: waywardmonkeys

system/include/libcxx/__config

@@ -648,6 +648,8 @@ template <unsigned> struct __static_assert_check {};
 #define _LIBCPP_ELAST __ELASTERROR
 #elif defined(__APPLE__)
 // Not _LIBCPP_ELAST needed on Apple
+#elif defined(__EMSCRIPTEN__)
+#define _LIBCPP_ELAST 256
 #else
 // Warn here so that the person doing the libcxx port has an easier time:
 #warning This platform's ELAST hasn't been ported yet