Seems to be being worked on, an attribute @_cdecl has been merged into master [0] and seems to compile in the current beta of 3.0 [1] although I haven't tested it.

[0] https://github.com/apple/swift/commit/013aad13d4245a012cfb76...

[1] http://swiftlang.ng.bluemix.net/#/repl/d68be430e72609717f718...