[lldb][Process/FreeBSDKernelCore] Fix RegisterContext for arm64 (#183947)
Since `pcb.lr` always contains the value of pc, `gpr_lr_arm64` should be
unavailable. This also fixes the case where `gpr_pc_arm64` displays sp
not lr field in pcb.
Reported by: jrtc27
Fixes: 4f0eb3d3af443ff54425ebafce83b87ee61ee403 (#180222)
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Signed-off-by: Minsoo Choo <minsoochoo0122@proton.me>