tests: kernel: Fix address-of-packed-mem warning

The warning below appears once -Waddress-of-packed-mem is enabled:

/__w/zephyr/zephyr/tests/kernel/mem_protect/userspace/src/main.c: In
function 'test_main':
/__w/zephyr/zephyr/tests/kernel/mem_protect/userspace/src/main.c:1024:17:
error: converting a packed 'k_thread_stack_t' {aka 'struct
z_thread_stack_element'} pointer (alignment 1) to a 'struct
z_x86_thread_stack_header' pointer (alignment 4096) may result in an
unaligned pointer value [-Werror=address-of-packed-member]
 1024 |  hdr = ((struct z_x86_thread_stack_header *)ztest_thread_stack);

To avoid the warning, use an intermediate void * variable.

More info in #16587.

Signed-off-by: Carles Cufi <[email protected]>

Author: carlescufi

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

@@ -1012,16 +1012,18 @@ void test_main(void)
 
 #if defined(CONFIG_ARM64)
 	struct z_arm64_thread_stack_header *hdr;
+	void *vhdr = ((struct z_arm64_thread_stack_header *)ztest_thread_stack);
 
-	hdr = ((struct z_arm64_thread_stack_header *)ztest_thread_stack);
+	hdr = vhdr;
 	priv_stack_ptr = (((char *)&hdr->privilege_stack) +
 			  (sizeof(hdr->privilege_stack) - 1));
 #elif defined(CONFIG_ARM)
 	priv_stack_ptr = (char *)z_priv_stack_find(ztest_thread_stack);
 #elif defined(CONFIG_X86)
 	struct z_x86_thread_stack_header *hdr;
+	void *vhdr = ((struct z_x86_thread_stack_header *)ztest_thread_stack);
 
-	hdr = ((struct z_x86_thread_stack_header *)ztest_thread_stack);
+	hdr = vhdr;
 	priv_stack_ptr = (((char *)&hdr->privilege_stack) +
 			  (sizeof(hdr->privilege_stack) - 1));
 #elif defined(CONFIG_RISCV)