Port `jnimarshalmethod-gen.exe` to net5

[jonpryor]

At present, tools/jnimarshalmethod-gen only builds against the net472 target framework. For .NET 5 support, we believe that we'd need to port this utility to run on .NET 5, and at present that would be by porting to the netcoreapp3.1 target framework.

Unfortunately, .NET Core currently doesn't support AssemblyBuilder.Save(): dotnet/runtime#15704

We will need to investigate if it's even possible to port jnimarshalmethod-gen.exe to .NET Core and .NET 5, assuming that the above issue is not fixed. (Note: above issue was opened in 2015…)

[jonpryor]

While AssemblyBuilder.Save() and AssemblyBuilderAccess.Save no longer exist, there is an AssemblyBuilderAccess.RunAndSave value, and the AssemblyBuilder docs do mention the ability to save IL to disk:

AssemblyBuilderAccess.RunAndSave is specified when the assembly is created. The assembly code is used immediately, and the assembly is also saved to disk so that it can be examined with Ildasm.exe (IL Disassembler) or used in another program.

This suggests that we can migrate jnimarshalmethod-gen.exe to net5 or net6…

…except their own example is using AssemblyBuilder.Save()! This is not particularly encouraging…

[jonpryor]

…oh, and AssemblyBuilderAccess.RunAndSave doesn't actually exist:

error CS0117: 'AssemblyBuilderAccess' does not contain a definition for 'RunAndSave'

Nope, still no workaround. Plus, obsolete docs!

[NickAcPT]

When I faced this exact issue on another project, I did a bit of research and it appears that one of the few workarounds is to use something like https://github.com/Lokad/ILPack.

Although I'm not quite sure if it can serve as a reliable drop-in workaround for jnimarshalmethod-gen