x86/setup: move CPU0s stack out of the Xen text/data/bss virtual region

x86/setup: move CPU0s stack out of the Xen text/data/bss virtual region

Currently, the BSP's stack is the BSS symbol cpu0_stack.  In builds using
memguard_stack(), a page gets shot out of the mappings.

To avoid shattering the superpage which will eventually map the BSS, use the
directmap virtual address of cpu0_stack, while still using the same underlying
physical memory.  (Xen has an order 21 physical relocation requirement meaning
that the order 3 alignment requirement for cpu0_stack will be honoured even
via its diretmap mapping.)

In addition, fix two issues exposed by the changes.

 * do_invalid_op() should use is_active_kernel_text() rather than having its
   own, different, idea of when to search through the bugframes.
 * Setting of system_state to active needs to be deferred until after code has
   left .init.text, for bugframes/backtraces to function in reinit_bsp_stack().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>