x86, 64bit, realmode: Use init_level4_pgt to set trampoline_pgd directly

with #PF handler way to set early page table, level3_ident will go away with
64bit native path.

So just use entries in init_level4_pgt to set them in trampoline_pgd.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-10-git-send-email-yinghai@kernel.org
Cc: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Acked-by: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>