compiler: allow arbitrary label string for JSFFI exports

This commit allows arbitrary label string to appear in a foreign
export declaration, as long as the calling convention is javascript.
Well, doesn't make sense to enforce it's a C function symbol for a
JSFFI declaration anyway, and it gets in the way of implementing the
"sync" flavour of exports.

(cherry picked from commit a204df3a)

compiler/GHC/Tc/Gen/Foreign.hs

@@ -78,7 +78,7 @@ import GHC.Data.Bag
 import GHC.Driver.Hooks
 import qualified GHC.LanguageExtensions as LangExt
 
-import Control.Monad ( zipWithM )
+import Control.Monad ( when, zipWithM )
 import Control.Monad.Trans.Writer.CPS
   ( WriterT, runWriterT, tell )
 import Control.Monad.Trans.Class
@@ -444,7 +444,7 @@ tcFExport d = pprPanic "tcFExport" (ppr d)
 tcCheckFEType :: Type -> ForeignExport GhcRn -> TcM (ForeignExport GhcTc)
 tcCheckFEType sig_ty edecl@(CExport src (L l (CExportStatic esrc str cconv))) = do
     checkCg (Left edecl) backendValidityOfCExport
-    checkTc (isCLabelString str) (TcRnInvalidCIdentifier str)
+    when (cconv /= JavaScriptCallConv) $ checkTc (isCLabelString str) (TcRnInvalidCIdentifier str)
     cconv' <- checkCConv (Left edecl) cconv
     checkForeignArgs isFFIExternalTy arg_tys
     checkForeignRes nonIOok noCheckSafe isFFIExportResultTy res_ty