hurd: Remove __hurd_local_reply_port

Now that the signal code no longer accesses it, the only real user of it
was mig-reply.c, so move the logic for managing the port there.

If we're in SHARED and outside of rtld, we know that __LIBC_NO_TLS ()
always evaluates to 0, and a TLS reply port will always be used, not
__hurd_reply_port0. Still, the compiler does not see that
__hurd_reply_port0 is never used due to its address being taken. To deal
with this, explicitly compile out __hurd_reply_port0 when we know we
won't use it.

Also, instead of accessing the port via THREAD_SELF->reply_port, this
uses THREAD_GETMEM and THREAD_SETMEM directly, avoiding possible
miscompilations.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>