syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

Currently, only x86 genuinely implements and supports Syscall User Dispatch
(SUD).

Multiple architectures provide a stub arch_syscall_is_vdso_sigreturn()
returning 'false' simply to satisfy GENERIC_ENTRY compilation, which
creates a false impression of feature support.

Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH to decouple this mechanism
from GENERIC_ENTRY. Select it exclusively on x86 and remove the redundant
stub functions from other architectures.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20260713035422.582771-1-ruanjinjie@huawei.com

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