Freebsd cross-compilation fails to link libexpr-c

[Mic92]

Describe the bug

$ nix develop .#packages.x86_64-linux.nix-x86_64-unknown-freebsd
$ ./configure $configureFlags
$ make

x86_64-unknown-freebsd-ld: error: undefined symbol: operator new(unsigned long, std::align_val_t)
>>> referenced by nix_api_expr.cc:108 (src/libexpr-c/nix_api_expr.cc:108)
>>>               src/libexpr-c/nix_api_expr.o:(nix_state_create)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [mk/lib.mk:84: src/libexpr-c/libnixexprc.so] Error 1

Is freebsd not linking libc++ correctly?

[Mic92]

cc @rhelmot @Ericson2314 who may have interest in this.

[Mic92]

@rhelmot says it's because of wrong llvm configuration and it might be fixed in master.
Will test this.

[Mic92]

Switching to nixpkgs unstable fixes it.

[Ericson2314]

The alignment of this is probably uninteresting, so we could write some fallback code too.