Automatically define preprocessor symbols for target platforms

[dsplaisted]

Spec is here: https://github.com/dotnet/designs/blob/master/accepted/2020/net5/net5.md#preprocessor-symbols

Note that this includes defining preprocessor symbols for the target platform, as well as defining preprocessor symbols for previous versions of .NET and the OS

[dsplaisted]

@sfoslund Here's a piece of the .NET 5 work that I think you could pick up and implement now, if you are looking for something.

We don't yet parse the TargetPlatformIdentifier or TargetPlatformVersion out of the TargetFramework, but the preprocessor symbol logic shouldn't care, and you can just set the target platform properties directly in tests.

[sfoslund]

#11635 implements target platform and NET backwards compatible preprocessor symbols. Outstanding work: adding backwards compatibility for target platforms.