linux: Add ptrace(2) support on arm64

This moves linux_ptrace.c from sys/amd64/linux/ to sys/compat/linux/,
making it possible to use it on architectures other than amd64.
It also enables Linux ptrace(2) on arm64.

Relnotes: yes
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32868