Make JIT configuration a single-init static (#2208)

Make JIT configuration a single-init static (#2208)

This fixes two things:
* UseJitPath on compilation builder doesn't make sense for any other backends this file is shared with (CppCodegen/LLVM). Moving that to ReadyToRunCodenCompilationBuilder.
* JitConfigProvider made it look like a configurable JIT path and configurable COMPlus RyuJIT options are a thing, but they're in fact per process and can only be initialized once (first is an implementation limitation in CorInfoImpl, the second is a RyuJIT limitation). Making the config class static+throwing for double-initialization to make this very clear.