Re-add CDECL calls

This addresses: https://dev.metasploit.com/redmine/issues/7639
Metasploit pull: https://github.com/rapid7/metasploit-framework/pull/1463

Adds CDECL calling functionality to Railgun.

A Calling Convention string is added to the Railgun TLV in metasploit-framework pull request.

Was pretty straightforward to define some new __cdecl functions in railgun.h and put an if statement on the calling convention. 

Not tested in x64 (cant compile with express), but shouldn't affect that functionality.