Support GOARCH constraints for the `syscall` package overlays.
Support GOARCH constraints for the `syscall` package overlays.

This mimics behavior of the build context for the upstream sources and
allows us to customize overlays for the darwin/arm64 platform.

gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs