arm64 makectx: Fix overflow of tf_x array

PCB_LR isn't stored in tf_x, so trying to store it as pcb_x[PCB_LR] =
tf->tf_x[PCB_LR + PCB_X_START] overflowed the tf_x array.

Reported by: Morello (bounds check crash)
Reviewed by: jrtc27, andrew, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D41485