cc-wrapper: Unconditionally warn about skipped native flags

[zhaofengli]

Description of changes

This PR makes cc-wrapper warn about skipped native flags (e.g., -march=native) unconditionally, so the behavior is more visible for users using the wrapped compilers in a Nix shell environment where they would expect such flags to work.

Currently, cc-wrapper strips out such flags silently when NIX_ENFORCE_NO_NATIVE is enabled (the default in stdenv). A message is only emitted if NIX_DEBUG is set. This makes sense for reproduciblility, but the behavior can be surprising/undesirable to users using it interactively. The compiler shouldn't silently not do what is specified without informing the user.

The downside of adding such an unconditional warning is log spam during builds that specify any native flag. However, we should fix the affected packages to not use such flags in the first place. Currently, a full build of stdenv with this PR applied does not emit such warnings.

Alternative solutions

1. Make cc-wrapper simply exit with a non-zero code when such flags are passed and NIX_ENFORCE_NO_NATIVE is enabled. This will probably break some packages, but is a cleaner solution.
2. Make the default mkShell environment not have NIX_ENFORCE_NO_NATIVE.

Workarounds

1. NIX_CFLAGS_COMPILE=-march=native produces the expected result since the flags are appended after filtering, but the effect pollutes the entire shell/build environment.

Things done

• Built on platform(s)
  • x86_64-linux (built stdenv on master)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[qweered]

@Ericson2314

[FliegendeWurst]

Seems like a good idea

[emilazy]

I think a hard failure on NIX_ENFORCE_NO_NATIVE might be a good idea, but this seems like a reasonable way to help gauge what the fallout of that might be.