Kestrel: add support for big endian systems

### Is your feature request related to a problem? Please describe.
I am trying to run .NET Core on an IBM i system (formerly known as AS/400). Installing Mono and running C# HelloWorld works like a charm. But when I try to get a .NET Core Web API application up and running, it fails with the message that Kestrel does not support big endian systems, which is quite disappointing, since I couldn't find any other .NET Core webservers for non-Windows systems.

### Describe the solution you'd like
Please add big endian support to Kestrel in order to enable full cross platform functionality, since that's what you would expect if you start development with cross platform technologies.

### Additional context
Java offers full support for big endian systems and that's exactly what I expect of .NET Core.