Delaying the runtime.throw until here will print more information.
In particular it will print the signal and code values, which means
it will show the fault address.
The canpanic checks were added recently, in CL 75320043.
They were just not added in exactly the right place.
LGTM=iant
R=dvyukov, iant
CC=golang-codereviews
https://golang.org/cl/83980043
SetPanicOnFault allows recovery from unexpected memory faults.
This can be useful if you are using a memory-mapped file
or probing the address space of the current program.
LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/66590044
