build: Use system default GOARCH, ignore GOARCH env var.
build: Use system default GOARCH, ignore GOARCH env var.

This way, when GOARCH env var is set to js, the syscall package
continues to build successfully.

Remove special case for runtime package. There was no comment
justifying why it needs to be special, I tried removing the special
case and it seems to work.

Helps #594.

gopherjs/gopherjs,gopherjs/gopherjs,albrow/gopherjs,gopherjs/gopherjs,albrow/gopherjs,gopherjs/gopherjs,albrow/gopherjs,gopherjs/gopherjs