StablePtr# array operations should be lev-polymorphic

StablePtr# is levity polymorphic: TYPE (BoxedRep lev) -> Type. Most primitive operations on StablePtr# are levity polymorphic, but array operations such as indexStablePtrArray#, readWord8ArrayAsStablePtr# etc. are not. As far as I understand, there is no reason for this limitation. The change should be

--- a/utils/genprimopcode/AccessOps.hs
+++ b/utils/genprimopcode/AccessOps.hs
@@ -24,7 +24,7 @@ elementTypes =
   , MkElementType "Addr"      (strToTy "Addr#"  ) "a machine address"              machWord
   , MkElementType "Float"     (strToTy "Float#" ) "a single-precision floating-point value"  (Just 4)
   , MkElementType "Double"    (strToTy "Double#") "a double-precision floating-point value"  (Just 8)
-  , MkElementType "StablePtr" (TyApp (TyCon "StablePtr#") [TyVar "a"])
+  , MkElementType "StablePtr" (TyApp (TyCon "StablePtr#") [TyVar "a_levpoly"])
                   "a 'StablePtr#' value"  machWord
   ] ++ do
   n <- [8, 16, 32, 64]

but it needs a test and CLC approval.

I have audited other operations in GHC.Prim. The only other case where we have a levity-monomorphic operation on a levity-polymorphic primitive are atomicModifyMutVar* operations. Since they rely on using a thunk, they can't be generalized.