BuildRustCrate: proc macros must be built for build's platform

When cross compiling proc macros, the proc macro needs to be built for the
build platform's architecture.

Without this change cross compiling from Darwin to Linux would simply
fail because it tries to link to a library with a file extension that
doesn't exist on the builder's platform.

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -49,6 +49,8 @@ let
           filename =
             if lib.any (x: x == "lib" || x == "rlib") dep.crateType
             then "${dep.metadata}.rlib"
+            # Adjust lib filename for crates of type proc-macro. Proc macros are compiled/run on the build platform architecture.
+            else if (lib.attrByPath [ "procMacro" ] false dep) then "${dep.metadata}${stdenv.buildPlatform.extensions.library}"
             else "${dep.metadata}${stdenv.hostPlatform.extensions.library}";
         in
         " --extern ${opts}${name}=${dep.lib}/lib/lib${extern}-${filename}"