Support PTRACE_GETREGSET w/ NT_PRSTATUS in Linux ptrace(2).

While Linux strace(1) doesn't strictly require it - it has a fallback
to PTRACE_GETREGS - it's a newer interface, so we better support it
before the old one is deprecated.

Reviewed by: dchagin
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20152