Add Debug conditional for Write-Host logging in VSTS Powershell scripts

https://github.com/xamarin/xamarin-macios/pull/8327

We have a lot of debugging "print" statements in Powershell files. The pipeline already has access to the following global variable:

```
if [[ "$SYSTEM_DEBUG" == "true" ]]; then
DEBUG="-d"
else
DEBUG=""
fi
```

It should be easy to incorporate a check for the variable into Powershell code so that we do not always execute debugging statements.