linux64: add arm64 linuxulator build details

The arm64 linuxulator needs different arguments for the objcopy
invocation used to build the linux VDSO. These arguments are both arch-
and OS-dependent, so I did not try to use some common setting for them.

Reviewed by: imp
Sponsored by: Turing Robotic Industries
Differential Revision: https://reviews.freebsd.org/D16011