Fix multiple projects for optimizations, debug/trace, etc.

System.Linq.Parallel.csproj wasn't outputting symbols, and it didn't have optimizations enabled in Release.

System.Numerics.Vectors.csproj was setting TRACE in Release builds but not in Debug builds.

System.Text.RegularExpressions.csproj wasn't setting either DEBUG nor RELEASE constants in either Debug/Release builds, it didn't have optimizations enabled in Release, and it wasn't generating symbols in either.

The XML libraries were't setting DEBUG/TRACE in either Debug/Release builds, weren't enabling optimizations in Release builds, and weren't generating symbols in either.

DEBUG is important in almost all of these libraries, as they use Debug.* for asserts and the like.  Symbol generation is important for debugging.  Optimizations are obviously important (but just to verify I saw significant performance wins in multiple libraries just from turning this on in release).  TRACE isn't really important, as none of the libraries do release tracing, but I thought it good hygiene to enable it.
My question was more generally: what is Silverlight even doing in here? I only see it being used in [XHashtable.cs](https://github.com/dotnet/corefx/blob/62ff891779a9510903c3b72aa5ff7a7c085fb85c/src/System.Xml.XDocument/src/System/Xml/Linq/XHashtable.cs) and [XmlWellformedWriter.cs](https://github.com/dotnet/corefx/blob/62ff891779a9510903c3b72aa5ff7a7c085fb85c/src/System.Xml.XPath.XmlDocument/src/System/Xml/XmlWellformedWriter.cs), should it probably be removed?

Or are there plans to rebuild SL from corefx sources? :)