Add ldOverrideWhitelist to only default to ldOverride on windows/linux

On some platforms - ie darwin, javascript etc we really do not want to
allow the user to use any linker other than the default one as this
leads to all kinds of bugs. Therefore it is a bit more prudant to add a
whitelist which specifies on which platforms it might be possible to use
a different linker.

utils/ghc-toolchain/exe/Main.hs

@@ -309,6 +309,15 @@ archHasNativeAdjustors = \case
   _          -> False
 
 
+-- | The platforms which we attempt to override ld
+ldOverrideWhitelist :: ArchOS -> Bool
+ldOverrideWhitelist a =
+  case archOS_OS a of
+    OSLinux   -> True
+    OSMinGW32 -> True
+    _ -> False
+
+
 mkTarget :: Opts -> M Target
 mkTarget opts = do
     -- Use Llvm target if specified, otherwise use triple as llvm target
@@ -320,7 +329,7 @@ mkTarget opts = do
     (archOs, tgtVendor) <- parseTriple cc0 (optTriple opts)
     cc <- addPlatformDepCcFlags archOs cc0
     readelf <- optional $ findReadelf (optReadelf opts)
-    ccLink <- findCcLink tgtLlvmTarget (optCcLink opts) (fromMaybe True (optLdOverride opts)) archOs cc readelf
+    ccLink <- findCcLink tgtLlvmTarget (optCcLink opts) (fromMaybe (ldOverrideWhitelist archOs) (optLdOverride opts)) archOs cc readelf
 
     ar <- findAr tgtVendor (optAr opts)
     -- TODO: We could have