Implement in-kernel relocator for the arm64 module linker.

It is decided to go with the shared object file format for modules on
arm64, due to the Aarch64 instruction set details. Combination of the
signed 28-bit offset in the branch instructions encoding together with
the supported memory model of compilers makes the relocatable object
support impossible or at least too hard.

Reviewed by: andrew, emaste
Tested by: andrew (on real hardware)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3796