hurd: Fix aligning signal stack pointer

Fixes 60f9bf97
"hurd: Port trampoline.c to x86_64"

Checked on x86_64-gnu.

Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230515083323.1358039-2-bugaevc@gmail.com>