linux: Make linux_ptrace.c portable

Make sys/amd64/linux/linux_ptrace.c machine-independent,
in preparation for moving it into sys/compat/linux/.
No functional changes.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32756

(cherry picked from commit c91d0e59be10f9412cbce467b8b038f006bb6f18)