Duplicate cxplat code

[dthaler]

microsoft/cxplat: Cross platform, C abstraction layer for both user mode and kernel mode libraries

usersim/cxplat at main · microsoft/usersim

We should not have two separate cxplat code bases.

[mtfriesen]

Three, if you count the original cxplat codebase in https://github.com/microsoft/msquic/tree/main/src/platform. The plan was to migrate that directory (minus the QUIC-specific code that leaked in) over to https://github.com/microsoft/cxplat.