treewide: All the linker to be chosen independently

This will begin the process of breaking up the `useLLVM` monolith. That
is good in general, but I hope will be good for NetBSD and Darwin in
particular.

Co-authored-by: sterni <sternenseemann@systemli.org>

lib/systems/default.nix

@@ -41,6 +41,19 @@ rec {
         else if final.isNetBSD              then "nblibc"
         # TODO(@Ericson2314) think more about other operating systems
         else                                     "native/impure";
+      # Choose what linker we wish to use by default. Someday we might also
+      # choose the C compiler, runtime library, C++ standard library, etc. in
+      # this way, nice and orthogonally, and deprecate `useLLVM`. But due to
+      # the monolithic GCC build we cannot actually make those choices
+      # independently, so we are just doing `linker` and keeping `useLLVM` for
+      # now.
+      linker =
+        /**/ if final.useLLVM or false      then "lld"
+        else if final.isDarwin              then "cctools"
+        # "bfd" and "gold" both come from GNU binutils. The existance of Gold
+        # is why we use the more obscure "bfd" and not "binutils" for this
+        # choice.
+        else                                     "bfd";
       extensions = {
         sharedLibrary =
           /**/ if final.isDarwin  then ".dylib"