Template Haskell only supports the CCall and StdCall calling conventions, while GHC supports more:

http://git.haskell.org/packages/template-haskell.git/blob/9bcc122819a6f4a2ae7ad569717324b8368e801c:/Language/Haskell/TH/Syntax.hs\#l1311

http://git.haskell.org/ghc.git/blob/7369d2595a8cceebe457a44c8400828f4df87ea0:/compiler/prelude/ForeignCall.lhs\#l159

CApiConv, PrimCallConv and JavaScriptCallConv are missing, and I think they should just be added unless this is expected to lead to problems.

I'd personally like to have JavaScriptCallConv so I can remove the horrible hacks to make ghcjs-ffiqq (https://github.com/ghcjs/ghcjs-ffiqq) work. I imagine that PrimCallConv would be useful for a package using a similar approach for high performance inline assembly or LLVM with quasiquotes.

A small change in GHC required to convert the new constructors, and possibly to check if the required extension is enabled.