Ensure channels don't leak exception effects (#25318)

The forward declarations cause `Exception` to be inferred - also,
`llrecv` is an internal implementation detail and the type of the
received item is controlled by generics, thus the ValueError raised
there seems out of place for the generic api.

tests/threads/tmembug.nim

@@ -12,14 +12,14 @@ var
 chan1.open()
 chan2.open()
 
-proc routeMessage*(msg: BackendMessage) =
+proc routeMessage*(msg: BackendMessage) {.raises: [], gcsafe.} = # no exceptions!
   discard chan2.trySend(msg)
 
 var
   recv: Thread[void]
   stopToken: Atomic[bool]
 
-proc recvMsg() =
+proc recvMsg() {.raises: [], gcsafe.} = # no exceptions!
   while not stopToken.load(moRelaxed):
     let resp = chan1.tryRecv()
     if resp.dataAvailable: