nios2: implement architecture-specific portion of sys_clone3

nios2: implement architecture-specific portion of sys_clone3

This commit adds the sys_clone3 entry point for nios2. An
architecture-specific wrapper (__sys_clone3) is required to save and
restore additional registers to the kernel stack via SAVE_SWITCH_STACK
and RESTORE_SWITCH_STACK.

Signed-off-by: Simon Schuster <schuster.simon@siemens-energy.com>
Link: https://lore.kernel.org/20250901-nios2-implement-clone3-v2-4-53fcf5577d57@siemens-energy.com
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>