linux: make PTRACE_SETREGS use a correct struct

Note that this is largely untested at this point, as was
the previous version; I'm committing this mostly to get
rid of struct linux_pt_reg.

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32735

(cherry picked from commit f0d9a6a781f331440baf9a846e773e44a297d59c)