Two build failures during cross-compilation

[avnik]

Both failures are build of ocaml itself
I cut most interesting parts from logs (patching, detecting architectures, and failure itself), but could provide full logs if needed.
You can see, that patches from overlay not applied, and so build of cross compiler fails.

I isolated problem to minimal flake, which illustrating how I used it -- https://github.com/avnik/nixpkgs-cross-build-repro/tree/avnik/ocaml-cross-overlay-repro
Could you hint, if I used overlay in wrong way or help me debug this issue if usage of overlay is correct.
Will be grateful for your help.

unpacking sources
unpacking source archive /nix/store/121nmhs8zpd9729jhmskvsd6n010cwim-ocaml-4.14.1.tar.xz
source root is ocaml-4.14.1
setting SOURCE_DATE_EPOCH to timestamp 1671539204 of file ocaml-4.14.1/yacc/wstr.c
@nix { "action": "setPhase", "phase": "patchPhase" }
patching sources
@nix { "action": "setPhase", "phase": "updateAutotoolsGnuConfigScriptsPhase" }
updateAutotoolsGnuConfigScriptsPhase
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.sub
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.guess
@nix { "action": "setPhase", "phase": "configurePhase" }
configuring
fixing libtool script ./build-aux/ltmain.sh
./configure
configure flags: --disable-static -prefix /nix/store/dzn0inc8w11bs0lwh5x12nzapf74l323-ocaml+flambda-4.14.1 --enable-flambda --host=x86_64-unknown-linux-gnu --target=aarch64-unknown-linux-gnu
configure: Configuring OCaml version 4.14.1
checking build system type... x86_64-pc-linux-gnu
checking host system type... x86_64-unknown-linux-gnu
checking target system type... aarch64-unknown-linux-gnu
...
make[5]: Entering directory '/build/ocaml-4.14.1/api_docgen/ocamldoc'
mkdir -p build/
cp ../../api_docgen/Ocaml_operators.mld build/Ocaml_operators.mld
mkdir -p build/libref
../../boot/ocamlrun ./../../ocamldoc/ocamldoc -I ../../stdlib -I ../../otherlibs/str -I ../../otherlibs/unix -I ../../otherlibs/dynlink -I ../../otherlibs/systhreads -text build/Ocaml_operato>
Fatal error: cannot load shared library dllunix
Reason: dllunix.so: cannot open shared object file: No such file or directory
make[5]: *** [Makefile:60: build/libref/Ocaml_operators.odoc] Aborted (core dumped)

And build for target

unpacking source archive /nix/store/121nmhs8zpd9729jhmskvsd6n010cwim-ocaml-4.14.1.tar.xz
source root is ocaml-4.14.1
setting SOURCE_DATE_EPOCH to timestamp 1671539204 of file ocaml-4.14.1/yacc/wstr.c
@nix { "action": "setPhase", "phase": "patchPhase" }
patching sources
@nix { "action": "setPhase", "phase": "updateAutotoolsGnuConfigScriptsPhase" }
updateAutotoolsGnuConfigScriptsPhase
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.sub
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.guess
@nix { "action": "setPhase", "phase": "updateAutotoolsGnuConfigScriptsPhase" }
updateAutotoolsGnuConfigScriptsPhase
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.sub
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.guess
@nix { "action": "setPhase", "phase": "configurePhase" }
configuring
fixing libtool script ./build-aux/ltmain.sh
./configure
configure flags: --disable-static -prefix /nix/store/qhqzlxgw5i89pynx9mgwg6arv70r14x6-ocaml+flambda-aarch64-unknown-linux-gnu-4.14.1 --enable-flambda --host=aarch64-unknown-linux-gnu --target=aarch64-unknown-linux-gnu
configure: Configuring OCaml version 4.14.1
checking build system type... x86_64-pc-linux-gnu
checking host system type... aarch64-unknown-linux-gnu
checking target system type... aarch64-unknown-linux-gnu
...
aarch64-unknown-linux-gnu-gcc -c -O2 -fno-strict-aliasing -fwrapv -pthread -Wall -Wdeclaration-after-statement -fno-common -fexcess-precision=standard -ffunction-sections -g  -D_FILE_OFFSET_BITS=64 -DCAML_NAME_SPACE  -DCAMLDLLIMPORT=  -o startup_byt.b.o startup_byt.c
In file included from caml/alloc.h:23,
                 from startup_byt.c:32:
startup_byt.c: In function 'do_print_config':
caml/misc.h:382:32: error: too few arguments to function 'caml_stat_strdup'
  382 | #define caml_stat_strdup_of_os caml_stat_strdup
      |                                ^~~~~~~~~~~~~~~~
startup_byt.c:382:10: note: in expansion of macro 'caml_stat_strdup_of_os'
  382 |          caml_stat_strdup_of_os(OCAML_STDLIB_DIR));
      |          ^~~~~~~~~~~~~~~~~~~~~~
In file included from startup_byt.c:49:
caml/memory.h:161:29: note: declared here
  161 | CAMLextern caml_stat_string caml_stat_strdup(const char *s);
      |                             ^~~~~~~~~~~~~~~~
make[2]: *** [Makefile:366: startup_byt.b.o] Error 1

[anmonteiro]

I don't know if our cross-compilation support can work with import ... { system = ... }. You might be luckier using pkgsCross.aarch64_multiplatform{,_musl}.

[avnik]

@anmonteiro main problem -- we cross-compile whole system, not just few packages