syscall: Add call gate for get_environ_ptr

Now that the environment strings are stored as an array of strings,
they can be passed from the application via char **environ, which
is really defined as a function call to get_environ_ptr().

This works as is for flat build, but protected mode and kernel mode
require a call gate, which is added here.

include/sys/syscall_lookup.h

@@ -353,6 +353,7 @@ SYSCALL_LOOKUP(futimens,                   2)
 /* The following are defined only if environment variables are supported */
 
 #ifndef CONFIG_DISABLE_ENVIRON
+  SYSCALL_LOOKUP(get_environ_ptr,          0)
   SYSCALL_LOOKUP(clearenv,                 0)
   SYSCALL_LOOKUP(getenv,                   1)
   SYSCALL_LOOKUP(putenv,                   1)