stdenv/freebsd/cc: make overrideable fixing nixpkgs eval

[sternenseemann]

stdenv.cc didn't have an override attribute on FreeBSD previously,
breaking evaluation of all-packages.nix consequently.

Resolves #119075.

Not tested on FreeBSD yet, but nix-env -qaP -f ./. --argstr system x86_64-linux works again.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[sss123next]

nix-instantiate -A nix .                
error: cannot coerce null to a string, at /data/nixpkgs/pkgs/development/tools/build-managers/cmake/default.nix:61:18
(use '--show-trace' to show detailed location information)

[sternenseemann]

cmake not evaluating is also not good news, but anything not depending on cmake should work at least.

[sss123next]

 preConfigure = ''
    fixCmakeFiles .
    substituteInPlace Modules/Platform/UnixPaths.cmake \                                                                                                                                        
      --subst-var-by libc_bin ${lib.getBin stdenv.cc.libc} \
      --subst-var-by libc_dev ${lib.getDev stdenv.cc.libc} \
      --subst-var-by libc_lib ${lib.getLib stdenv.cc.libc}
  ''

this code block failing, looks like parser do not like '' for some reason

[sternenseemann]

Nope, it's because stdenv.cc.libc == null on FreeBSD because we don't package a libc for that platform (or it's not part of the stdenv at least).

[sss123next]

any idea how to fix or workaround it ?

[sternenseemann]

The best solution would be packaging FreeBSD's libc I think.

[sss123next]

as i understand, it's non-trivial task, as freebsd libc is a part of monolithic system core (builds alongside with kernel and basic set of tools similar to coreutils) i may be wrong o course ...

[sternenseemann]

See also #82131