When invoking a simple Ada program with gcc from gnats10, the
following warnings are shown:

$ gcc -c conftest.adb
gnat1: warning: command-line option ‘-Wformat=1’ is valid for C/C++/ObjC/ObjC++ but not for Ada
gnat1: warning: command-line option ‘-Wformat-security’ is valid for C/C++/ObjC/ObjC++ but not for Ada
gnat1: warning: ‘-Werror=’ argument ‘-Werror=format-security’ is not valid for Ada
$ echo $?
0

This is only spammy when compiling Ada programs inside a Nix derivation,
but certain configure scripts (such as the ./configure script from the
gcc that's built by coreboot's make crossgcc command) fail entirely
when getting that warning output.

https://nixos.wiki/wiki/Coreboot currently suggests manually running

NIX_HARDENING_ENABLE="${NIX_HARDENING_ENABLE/ format/}" make crossgcc

… but actually teaching the nixpkgs-provided cc wrapper that format
isn't supported as a hardening flag seems to be the more canonical way
to do this in nixpgks.

After this, Ada programs still compile:

$ gcc -c conftest.adb
$ echo $?
0

And the compiler output is empty.

cc @ajs124 due to the nixos wiki edit and @petabyteboy due to coreboot-utils :-)

This shouldn't be a scary rebuild, as the default gcc doesn't have ada support enabled.

Motivation for this change

Build Coreboot on NixOS

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.