Remove special treatment for ARM, fix native building

This is a further improvement on #1048, as none of this special treatment and redefinition of the go architecture is needed.  The goarch "arm" is not an invalid setting; exactly the opposite:
https://golang.org/doc/install/source#environment

When this is built on the appropriate compiler for the target ARM architecture, the software builds and runs as intended for that architecture.  If the intention was to provide a mechanism for cross-compiling, it should not interfere with normal, targeted builds.

As on x86, this allows native, targeted ARM builds to be completed with:
go run build.go -no-upgrade build