hurd: Rewrite __libc_cleanup_*

This makes it notably safe against 'return' and such, and used for
__libc_cleanup_push/pop.

	* sysdeps/mach/libc-lock.h (__libc_cleanup_frame): Define structure.
	(__libc_cleanup_fct): Define function.
	(__libc_cleanup_region_start, __libc_cleanup_region_end,
	__libc_cleanup_end): Rewrite implementation using
	__attribute__ ((__cleanup__)).
	(__libc_cleanup_push, __libc_cleanup_pop): New macros.