Don't use static on DPCPU data in arm64 modules

[andrew]

Summary

It is broken ad it doesn't emit a relocation for the in-kernel linker to
adjust. To work around this remove the static keyword from the
DPCPU_DEFINE_STATIC macro on arm64 when building modules.

PR: 225816

[bz]

Please also incorporate the one line subject summary into the description for the commit message.

sys/sys/pcpu.h

96 ↗

(On Diff #44930)

I think this should mention that this is an issue for file local static variables only and making them global the issue goes away.
Also, I'd remove the "arm64" from the comment and say "certain architectures" to keep it general as this might effect more of them?
I might also add a note that this is a "workaround" until better ways might be found?

[andrew]

Update the comment.