Fix vncdll on 64-bit systems.

[molecula2788]

This pull request introduces some fixes to vncdll in order to make it work on 64-bit systems:

• The ReflectiveLoader function in winvnc.dll will be compiled as a C++ function, and therefore its name will be mangled
• A DllMain needs to be present that handles DLL_METASPLOIT_ATTACH and calls Init
• Various changes where pointers were used as DWORDs, which breaks on 64-bit

[bwatters-r7]

Is this related to #20604?

[molecula2788]

That issue is only for the reverse_http stager.
From my tests, without the fixes in this pull request, the vncinject payload doesn't work at all in any configuration.

[dledda-r7]

as a side note, I am not sure how this dll was working before but we should have at least the Ordinal export instead of exporting the totally-not-sus ReflectiveLoader, kinda what we do with metsrv