arm64: Implement cpu_ptrace().

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

(cherry picked from commit d78c2cd831d346b94c00c72b5a94a1a4f0dd3e53)
Signed-off-by: Olivier Houchard <cognet@FreeBSD.org>