Don't use the static keyword with DPCPU defines in arm64 modules.

On arm64 compiler will create PC-relative loads and stores for static data.
This means it doesn't emit a relocation. Unfortunately the in-kernel linker
expects there to be one for DPCPU defines so it can modify its value so the
code will use the correct DPCPU region.

To workaround the lack of a relocation with static data remove it when
building modules on arm64. The kernel is unaffected as it doesn't rely on
modifying these relocations to find the data.

PR: 225684
Reported by: Johannes Lundberg <johalun0@gmail.com>
Reported by: Jose Luis Duran <jlduran@gmail.com>
Reported by: Greg V <greg@unrelenting.technology>
Reviewed by: bz
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D16145