From the original description in CL 15770043

The main change here is to consult $GOARCH.

In 6c, when GOOS=nacl, some of the more complex addressing modes must be disabled, and the BP and R15 registers must not be used.

See golang.org/s/go13nacl for design overview.

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/69020044