Do not clobber sp in _hurd_stack_setup.

GCC mainline now gives errors for an asm that clobbers the stack
pointer.  According to
<https://gcc.gnu.org/ml/gcc-patches/2018-12/msg00932.html> GCC
previously ignored such a clobber; thus, this patch removes it from
_hurd_stack_setup.

Tested with build-many-glibcs.py for i686-gnu.

	* sysdeps/mach/hurd/i386/init-first.c (_hurd_stack_setup): Do not
	clobber sp.