Ignore `purego` build constraint (#1767)

`purego` is a proposed[1] de-facto build constraint to
denote building _only_ Go code.

We must ignore this constraint to make sure that files
that toggle on `purego` are added to the Bazel target
`srcs` and let the compiler decide if they should be
compiled or not.

[1]: https://github.com/golang/go/issues/23172