Hadrian: avoid useless allocations in trackArgument

Cf ticky report before the change:

    Entries      Alloc    Alloc'd  Non-void Arguments      STG Name
--------------------------------------------------------------------------------
     696987   29044128          0   1 L                    main:Target.trackArgument_go5{v r24kY} (fun)

hadrian/src/Target.hs

+{-# LANGUAGE LambdaCase #-}
+
 module Target (
     Target, target, context, builder, inputs, outputs, trackArgument,
     module Builder
@@ -25,8 +27,14 @@ trackArgument target arg = case builder target of
     Cabal _ _ -> not $ verbosityArg arg || cabal_configure_ignore arg
     _         -> True
   where
-    threadArg s = dropWhileEnd isDigit s `elem` ["-j", "MAKEFLAGS=-j", "THREADS="]
-    verbosityArg s = dropWhileEnd isDigit s == "-v"
+    match_str_num []     rs     = all isDigit rs
+    match_str_num (x:xs) (r:rs) = x == r && match_str_num xs rs
+    match_str_num (_:_)  []     = False
+
+    threadArg s = match_str_num "-j" s || match_str_num "MAKEFLAGS=-j" s || match_str_num "THREADS=" s
+    verbosityArg s = match_str_num "-v" s
     diagnosticsColorArg s = "-fdiagnostics-color=" `isPrefixOf` s -- N.B. #18672
-    cabal_configure_ignore s =
-      s `elem` [ "--configure-option=--quiet", "--configure-option=--disable-option-checking" ]
+    cabal_configure_ignore = \case
+      "--configure-option=--quiet"                   -> True
+      "--configure-option=--disable-option-checking" -> True
+      _ -> False