Revert "riscv: PMP-based stack guard is incompatible with stack sentinel"

This reverts commit a7b5d606.

The assumption behind that commit was wrong. The software-based stack
sentinel writes to the very bottom of the _writable_ stack area i.e.
right next to the actual PMP based guard area. So they are compatible.

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