We cannot let a real panic start there, because there is C code
on the stack, and worse, there is an assembly frame with a
saved copy of the registers and we have no idea which ones
are pointers.

Instead, detect the nil ptr load/store and return out of the C
and assembly into a stub that will start the call to sigpanic.

Fixes GOARM=5 build.

LGTM=iant
R=golang-codereviews, iant
CC=dave, golang-codereviews, minux, r
https://golang.org/cl/138130043