Make the loading of llvm EH info async safe. (#4725)

Make the loading of llvm EH info async safe. (#4725)

* [aot] Emit class references in EH info in such as way that they can be skipped when reading them in async context.

* [aot] Rewrite the LLVM EH info loading code by moving allocations to callers so the lower level code is async safe.

* [aot] Make the memory allocation in the loading of llvm EH info async safe. Remove fields from MonoLLVMFDEInfo which are passed as parameters.

* [runtime] Add the ability to store the unwind info address/length in the MonoJitInfo structure instead of referencing it using the 'unwind_info' field.

* [aot] Make the loading of llvm EH information async safe.