x86-64, docs, mm: Add vsyscall range to virtual address space layout

Add the end of the virtual address space to its layout documentation.

Signed-off-by: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/1362428180-8865-4-git-send-email-bp@alien8.de


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>