the netcoreapp3.0 build is broken

[eerhardt]

Follow the instructions: https://github.com/dotnet/machinelearning/blob/master/docs/building/netcoreapp3.0-instructions.md

You will get a build error:

Error	CS0266	Cannot implicitly convert type 'float*' to 'int'. An explicit conversion exists (are you missing a cast?)	Microsoft.ML.CpuMath(netcoreapp3.0)	F:\git\machinelearning2\src\Microsoft.ML.CpuMath\SseIntrinsics.cs	758	Active

This is because of #1177

                float* pDstEnd = pdst + dst.Length;
                float* pDstCurrent = pdst;
                int destinationEnd = pDstEnd - 4;

pDstEnd is a float*. Subtracting 4 still gives you a float*, which is not convertable to int.

/cc @jwood803 @tannergooding

[jwood803]

@eerhardt Whoops! Sorry about that! I'll get this fixed.

[eerhardt]

Resolved by #1497