riscv: pmp: fix stackguard when used on SMP

The IRQ stack in particular is different on each CPU, and so is its
stack guard PMP entry value. This creates 2 issues:

- The assertion ensuring the last global PMP address is the same
  for each CPU does fail;

- That last global PMP address can't be relied upon to create a
  single-slot per-thread TOR mapping.

Fix both issues by not remembering the actual address for the last
global entry but a dummy address instead that is guaranteed not to
match any opportunistic single-slot TOR mapping.

While at it, lock that IRQ stack guard PMP entry.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>