alpha: provide ftrace return address support for lockdep

Lockdep uses ftrace_return_address() to report useful call sites for
lock acquisition and IRQ-state tracking diagnostics. Provide the Alpha
architecture hook using the compiler return-address builtin when frame
pointers are available.

Return zero when frame pointers are disabled, matching the existing
fallback behavior of architectures that cannot provide a reliable return
address.

This is a preparatory change for enabling lockdep support on Alpha.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20260706170019.2941459-5-linmag7@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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