Make Cmm Lint messages use dump style

Lint errors indicate an internal error in GHC, so it makes sense to use
it instead of the user style. This is consistent with Core Lint and STG Lint:

https://gitlab.haskell.org/ghc/ghc/-/blob/22096652/compiler/GHC/Core/Lint.hs#L429

https://gitlab.haskell.org/ghc/ghc/-/blob/22096652/compiler/GHC/Stg/Lint.hs#L144

Fixes #22218.

compiler/GHC/Cmm/Lint.hs

@@ -50,7 +50,8 @@ cmmLintGraph platform g = runCmmLint platform lintCmmGraph g
 runCmmLint :: OutputableP Platform a => Platform -> (a -> CmmLint b) -> a -> Maybe SDoc
 runCmmLint platform l p =
    case unCL (l p) platform of
-     Left err -> Just (vcat [text "Cmm lint error:",
+     Left err -> Just (withPprStyle defaultDumpStyle $ vcat
+                            [text "Cmm lint error:",
                              nest 2 err,
                              text "Program was:",
                              nest 2 (pdoc platform p)])