alpha: fill in pc for software event samples

Context switch and tracepoint events have no trap frame, so perf builds
one with perf_fetch_caller_regs(). Alpha does not define
perf_arch_fetch_caller_regs(), which leaves that pt_regs zeroed, and
these samples report pc as 0.

Record the caller's address as pc. The zero ps already marks the frame
as kernel mode, and pt_regs has nowhere to keep sp, so pc is all that
can be filled in.

perf_fetch_caller_regs() gets that address from CALLER_ADDR0, which
alpha defines as 0 when CONFIG_FRAME_POINTER is disabled. Finding the
level zero return address does not need a frame pointer, so make
alpha_ftrace_return_address0() available in both configurations. Only
ftrace_return_address(n) still depends on frame pointers.

Reported-by: Magnus Lindholm <linmag7@gmail.com>
Closes: https://lore.kernel.org/all/CA%2B%3DFv5S8-Zk8-LUpZnWiBfTGSQXthepmRc_-a9O%2BpEPmVW0mRA@mail.gmail.com/
Suggested-by: Magnus Lindholm <linmag7@gmail.com>
Assisted-by: Claude:claude-opus-5-5
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Acked-by: Ian Rogers <irogers@google.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20261018-alpha-perf-regs-v3-v3-5-52be85332c57@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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