entry: Split generic entry into generic exception and syscall entry

Currently CONFIG_GENERIC_ENTRY enables both the generic exception
entry logic and the generic syscall entry logic, which are otherwise
loosely coupled.

Introduce separate config options for these so that architectures can
select the two independently. This will make it easier for
architectures to migrate to generic entry code.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/20250213130007.1418890-2-ruanjinjie@huawei.com
Link: https://lore.kernel.org/all/20250624-generic-entry-split-v1-1-53d5ef4f94df@linaro.org

[Linus Walleij: rebase onto v6.16-rc1]