Add a minimal implementation of cpu_ptrace() for arm64. It is only used to
get/set VFP registers for 32bits binaries, as it is apparently what we use
there, instead of the MI PT_GETFPREGS/PT_SETFPREGS.
PR: 267361
MFC After: 1 week
set of machine headers needed to build the userland toolchain.
Differential Revision: https://reviews.freebsd.org/D2148
Reviewed by: imp
Sponsored by: The FreeBSD Foundation