hurd: Use __mach_setup_thread_call ()

...instead of mach_setup_thread (), which is unsuitable for setting up
function calls.

Checked on x86_64-gnu: the signal thread no longer crashes upon trying
to process a message.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230517191436.73636-6-bugaevc@gmail.com>