linux: mv sys/i386/linux/linux_ptrace{,_machdep}.c

In preparation for machine-independent sys/compat/linux/linux_ptrace.c,
rename the i386-specific Linux ptrace(2) implementation. No functional
changes.

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32757

(cherry picked from commit 4dfd612286ec2ecc7a05f3d4fd65a58f4b05f74c)