posix: implement mprotect()

Provide a stub for mprotect() to satisfy the requirement for the
base definitions / system interfaces and subsequently PSE51,
PSE52, PSE52, etc.

Currently, Zephyr's virtual memory-management API does not seem
to support modifying memory protection bits after pages have
already been mapped.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>