Harmonize the `show` methods for `Platform`
They are harmonized in both their implementation and their behavior:
both methods now show the tags in sorted order, whereas previously only
the method for `MIME("text/plain")` did so. This provides consistency
but also protects against hypothetical, unexpected breakage in the form
of downstream package tests relying on how things are printed should the
iteration order for `Dict`s change.
Also fix the case of no additional tags, which previously included a
semicolon followed by no keyword arguments, e.g.
```
julia> show(Platform("x86_64", "linux"))
Platform("x86_64", "linux"; )
```
which makes me insane. This now shows as `Platform("x86_64", "linux")`.
I'm still insane but at least the printing is nicer.
