runtime: turn divide by zero, nil dereference into panics

tested on linux/amd64, linux/386, linux/arm, darwin/amd64, darwin/386.
freebsd untested; will finish in a separate CL.

for now all the panics are errorStrings.
richer structures can be added as necessary
once the mechanism is shaked out.

R=r
CC=golang-dev
https://golang.org/cl/906041