bring back ___environ symbol (3 underscores)

its existence doesn't hurt anything, and dynamic-linked binaries using
previous versions of musl were wrongly binding to it instead of
__environ.

src/env/__environ.c

@@ -2,5 +2,6 @@
 
 #undef environ
 char **__environ = 0;
+weak_alias(__environ, ___environ);
 weak_alias(__environ, _environ);
 weak_alias(__environ, environ);