retire arm64 kernel module linker workaround

Relocatable linking in aarch64 ld from binutils 2.25.1 does not work.
The linker corrupts the references to the external symbols which are
defined by other object in the linking set and should therefore lose
the GOT entry.

The problem is fixed in later versions of GNU ld and does not exist in
the in-tree lld linker that we now use by default for arm64, so the
workaround can be removed.

Reviewed by: kib
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11302