hurd: Fix x86_64 sigreturn restoring bogus reply_port

Since the area of the user's stack we use for the registers dump (and
otherwise as __sigreturn2's stack) can and does overlap the sigcontext,
we have to be very careful about the order of loads and stores that we
do. In particular we have to load sc_reply_port before we start
clobbering the sigcontext.

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