Add System.Runtime.CompilerServices.Unsafe

[OwnageIsMagic]

https://github.com/dotnet/runtime/tree/main/src/libraries/System.Runtime.CompilerServices.Unsafe/src

exact commit in case it moved

https://github.com/dotnet/runtime/tree/053a472be0143bda0891c715d7b2ce7398538127/src/libraries/System.Runtime.CompilerServices.Unsafe/src

to build without configuring global.json replace

<Project Sdk="Microsoft.NET.Sdk.IL">

with

<Project Sdk="Microsoft.NET.Sdk.IL/6.0.0">