mandoc: fix executableCross condition

isCompatible concerns itself with architecture compatibility which does
not constitute executability by itself, since there are other key
factors like the Kernel/syscall interface targeted. As mode switching is
considered compatible (e.g. ppc64le and ppc64), we can't even use
isCompatible to establish the architecture subset of executability.
For the lack of a better alternative, we'll just compare the system
tuples for now which will loose us support for x86_64-linux ->
i686-linux, but this is probably a bearable consequence.

pkgs/tools/misc/mandoc/default.nix

@@ -5,7 +5,11 @@ let
   # even though the platforms aren't the same. mandoc can't be cross compiled
   # (easily) because of its configurePhase, but we want to allow “native” cross
   # such as pkgsLLVM and pkgsStatic.
-  executableCross = stdenv.hostPlatform.isCompatible stdenv.buildPlatform;
+  # For a lack of a better predicate at the moment, we compare the platforms'
+  # system tuples. See also:
+  # * https://github.com/NixOS/nixpkgs/pull/140271
+  # * https://github.com/NixOS/nixpkgs/issues/61414
+  executableCross = stdenv.buildPlatform.system == stdenv.hostPlatform.system;
 
   # Name of an UTF-8 locale _always_ present at runtime, used for UTF-8 support
   # (locale set by the user may differ). This would usually be C.UTF-8, but