mach: Add __mach_setup_thread_call ()

This is just like mach_setup_thread (), but it's suitable for making the
thread call a function correctly, as opposed to explicitly setting the
thread's stack and instruction pointers to the given values. Internally,
it uses MACHINE_THREAD_STATE_SETUP_CALL.

Unlike mach_setup_thread (), which is exported via mach.h for the
benefit of the Hurd exec server, __mach_setup_thread_call () is private
to glibc for the time being.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230517191436.73636-5-bugaevc@gmail.com>