x86: refine link time stub area related assertion

x86: refine link time stub area related assertion

While it has been me to introduce this, the use of | there has become
(and perhaps was from the very beginning) misleading. Rather than
avoiding the right side of it when linking the xen.efi intermediate file
at a different base address, make the expression cope with that case,
thus verifying placement on every step.

Furthermore the original check was too strict: We don't use one page per
CPU, so account for this as well. This involves moving the
STUBS_PER_PAGE definition and making DIV_ROUND_UP() accessible from
assembly (and hence the linker script); move a few other potentially
generally useful definitions along with it.

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