Support calls that require virtual stub dispatch.

[pgavlin]

VSD requires a special calling convention that places the target stub in a target-specific register.

[AndyAyersMS]

@pgavlin Can you link (if possible) to the part of the CoreCLR that describes what we need to do? I looked for something and it wasn't obvious to me.

[pgavlin]

The closest thing to documentation that I can find is the definition of the VSD stub itself.

Also relevant:

• The definition of REG_VIRTUAL_STUB_PARAM
• The definition of VSD_ResolveWorker

[pgavlin]

Also, the code in RyuJIT that seems to deal with this is here.

[AndyAyersMS]

Great, thanks. Looks like there are (at least) two other cases like this where some magic bits are passed in R11 and we have to handle a non-standard calling convention.

[pgavlin]

@AndyAyersMS It looks like this supporting this will require changes to LLVM. The easiest way to get this done appears to be to add a custom calling convention (I can't see any other straightforward way to place a particular value in a particular register), which AFAICT must be done by updating the various *CallingConv.td files.

[AndyAyersMS]

Completed via 87ee4e6 and matching changes over in LLVM.