* Add `monowasm` build target to reference assemblies.
* Cleanup unused reference.
* Update the WebAssembly.Net.Http.extra.cs to match the actual method signature.
* Remove System.Memory.dll
* Remove System.Memory
Extract the logic for building the Mono assemblies into
a separate directory instead of building in each .NET profile.
Add Mono.Options.dll and Mono.C5.dll, they weren't consistently
available in all profiles.
Also change the other Makefiles so assemblies don't depend
on the Makefile and bare/.stamp as it'd cause needless rebuilds
in cases where the repo is freshly checked out.