Fix dtrace fbt return probes on arm64

As with arm and riscv fix return fbt probes on arm64. arg0 should be
the offset within the function of the return instruction and arg1
should be the return value.

Reviewed by: kp, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33440

(cherry picked from commit e3ccf4f9dee9ae39774639407264199a07fa41ff)