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.

(cherry picked from commit 91febf1f4c)

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: