copper: broken on darwin

[stephank]

Motivation for this change

ZHF: #122042
cc @NixOS/nixos-release-managers

Hydra failure: https://hydra.nixos.org/build/142620896/nixlog/1

When setting CC to clang:

building
file-unix.c
Bootstrapping...
boot/copper-c-unix-64.c:1906:18: error: second parameter of 'main' (argument array) must be of type 'char **'
extern int CDECL main(int zargc, unsigned char **zargv);
                 ^
boot/copper-c-unix-64.c:3332:23: warning: declaration of built-in function 'fopen' requires inclusion of the header <stdio.h> [-Wbuiltin-requires-header]
extern FILE176 *CDECL fopen(unsigned char *, unsigned char *);
                      ^
boot/copper-c-unix-64.c:3334:21: warning: declaration of built-in function 'fread' requires inclusion of the header <stdio.h> [-Wbuiltin-requires-header]
extern size_t CDECL fread(unsigned char *, size_t , size_t , FILE176 *);
                    ^
boot/copper-c-unix-64.c:3335:21: warning: declaration of built-in function 'fwrite' requires inclusion of the header <stdio.h> [-Wbuiltin-requires-header]
extern size_t CDECL fwrite(unsigned char *, size_t , size_t , FILE176 *);
                    ^
boot/copper-c-unix-64.c:3837:11: error: second parameter of 'main' (argument array) must be of type 'char **'
int CDECL main(int zargc, unsigned char **zargv)
          ^
3 warnings and 2 errors generated.
make: *** [Makefile:131: boot] Error 1

When adding gcc as build input:

Linking...
ld: warning: ignoring file stage2/copper-elf64.o, building for macOS-x86_64 but attempting to link with file built for unknown-unsupported file format ( 0x7F 0x45 0x4C 0x46 0x02 0x01 0x01 0x03 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 )
Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
make[1]: *** [Makefile:147: stage2/copper-elf64] Error 1

(Not sure if clang works on other platforms.)

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.

[rmcgibbo]

cc also: #122842 (comment)

[veprbl]

Technically, it's not broken, but plain unsupported, but describing that would be too much effort.