runtime: handle nil ptr load/store in arm software floating point

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://codereview.appspot.com/138130043
misc/cgo/test: make issue5548 test pickier

If there is doubt about passing arguments correctly
(as there is in this test), there should be doubt about
getting the results back intact too. Using 0 and 1
(especially 0 for success) makes it easy to get a PASS
accidentally when the return value is not actually
being propagated. Use less common values.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://codereview.appspot.com/141110043