[BUG] perf: libdw not working for data collected on a different arch than host

[Shimin Guo <shimin.guo@skydio.com>]

Hi,

I see there have been patches aimed at enabling perf to unwind stacks
collected on a different arch than the host perf itself is running on,
for example, https://lore.kernel.org/all/20230606014559.21783-4-leo.yan@linaro.org/.

I believe there is still one issue that prevents me from doing
unwinding on arm64 stacks on a x86 host. As of v6.18-rc6, I still see
the following in tools/perf/util/unwind-libdw.c:

static const Dwfl_Thread_Callbacks callbacks = {
    .next_thread = next_thread,
    .memory_read = memory_read,
    .set_initial_registers = libdw__arch_set_initial_registers,
};

libdw__arch_set_initial_registers is defined for each architecture,
and the version is chosen at compile time, not for the arch the data
is collected on. After fixing this issue, I was able to get the call
chains. Is my analysis correct?

Best,
Shimin