arm64: cpu_switch: don't zero out pcb_onfault

Previously this would zero out x18 in the pcb, now it's attacking the
innocent pcb_onfault -- drop it entirely.

This technically fixes
e605b87a9e7 ("Save only callee-saved registers in pcb"), but it's
harmless until the below commit trims down pcb_x.

Reported by: mmel
Reviewed by: andrew, mmel
Fixes: 1c1f31a5e517 ("Remove unused registes from the arm pcb")
Differential Revision: https://reviews.freebsd.org/D39277