Fix abbreviation of channel ends in the C backend.

I'm pretty sure this test used to fail and got fixed, but I think the
test was fixed to match the incorrect code rather than the other way
around. What it's testing is the abbreviation of a channel end as an
identically-typed channel end, which (since they're both pointers)
should look something like:

Channel *c;
Channel *d = c;

Instead it was generating "d = &c", which broke cgtest87.

The change made to GenerateC to fix this feels a bit awkward to me --
it's very simple, but it's yet another special case in genVariableAM.
All of the genVariable code could really use reworking (again),
perhaps to use some internal representation of the C expression...

backends/GenerateC.hs

@@ -1133,6 +1133,7 @@ cgenVariableAM v am
                case (indirectedType t, t) of
                  (True, _) -> return ()
                  (False, A.Array {}) -> return ()
+                 (False, A.Chan {}) -> return ()
                  _ -> tell ["&"]
           call genVariable v
 

backends/GenerateCTest.hs

@@ -623,7 +623,7 @@ testSpec = TestList
   ,testAllSameForTypes 610 (\t -> ("$(" ++ show t ++ ")*const foo=(&bar);","")) (\t -> A.Is emptyMeta A.Abbrev t (variable "bar")) [chanInt,A.Record foo]
   --Abbreviations of channel-ends in C++ should just copy the channel-end, rather than trying to take the address of the temporary returned by writer()/reader()
   --C abbreviations will be of type Channel*, so they can just copy the channel address.
-  ,testAllForTypes 620 (\t -> ("$(" ++ show t ++ ") foo=&bar;","")) (\t -> ("$(" ++ show t ++ ") foo=bar;",""))
+  ,testAllForTypes 620 (\t -> ("$(" ++ show t ++ ") foo=bar;","")) (\t -> ("$(" ++ show t ++ ") foo=bar;",""))
     (\t -> A.Is emptyMeta A.Abbrev t (variable "bar")) [chanIntIn,chanIntOut]
   
   ,testAllSameForTypes 700 (\t -> ("const $(" ++ show t ++ ") foo=bar;","")) (\t -> A.Is emptyMeta A.ValAbbrev t (variable "bar")) [A.Int,A.Time]