tests: mem_protect/userspace: fix privilege stack ptr for Xtensa

The priv_stack_ptr for reading and writing tests was not set
correctly on Xtensa, and was always 0. Test would pass since
the NULL page should always generate access faults on most
Xtensa platforms. However, those tests are supposed to test
reading and writing to the privilege stack. So fix that by
populating the priv_stack_ptr correctly.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

tests/kernel/mem_protect/userspace/src/main.c

@@ -1191,6 +1191,12 @@ void *userspace_setup(void)
 	priv_stack_ptr = (char *)((uintptr_t)ztest_thread_stack +
 				  Z_RISCV_STACK_GUARD_SIZE);
 #endif
+#elif defined(CONFIG_XTENSA)
+	struct xtensa_thread_stack_header *hdr;
+	void *vhdr = ((struct xtensa_thread_stack_header *)ztest_thread_stack);
+
+	hdr = vhdr;
+	priv_stack_ptr = (((char *)&hdr->privilege_stack) + (sizeof(hdr->privilege_stack) - 1));
 #endif
 	k_thread_access_grant(k_current_get(),
 			      &test_thread, &test_stack,