ImageMagick: Silence flood of deprecation warnings from GCC

[YoshiRulz]

Description of changes

When building ImageMagick there's a warning on seemingly every source file:

cc1: warning: '-mtune=x86-64' is deprecated; use '-mtune=k8' or '-mtune=generic' instead as appropriate [8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wdeprecated-Wdeprecated8;;]

...obscuring "real" warnings. This PR applies the suggested fix of s/-mtune=x86-64/-mtune=generic/.

Resubmission of #195757 to target branch staging. Apologies for the notif spam. GitHub should really put a warning on that.
And to answer @AndersonTorres' question, I believe this is Nix-specific as the warning didn't appear during a manual build on my Manjaro machine nor in upstream's CI (Ubuntu Docker image).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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
• Fits CONTRIBUTING.md.

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[rhendric]

I've been looking into this PR, and I think there's something dodgy about the way nixpkgs is using --with-gcc-arch that should be fixed instead of patching configure as this PR does.

Per the relevant documentation, the --with-gcc-arch flag here drives -mtune, not -march as one might assume (ImageMagick is calling this macro with [yes]). The values recognized by -mtune, as was previously noted in #125150, do not include aarch64, and now it appears that x86-64 is also deprecated.

The --with-gcc-arch-using code seems to have been copied over from the ImageMagick 6 file, in which it was added nine years ago in 6f53886. However, the use of the AX_GCC_ARCHFLAG macro in ImageMagick was switched from [no] to [yes] in ImageMagick/ImageMagick@c7329a2 eight years ago, meaning that when our illustrious elder wrote that arch code it actually did affect -march and not -mtune.

I think the right thing to do here is drop the arch declaration and replace the lib.withFeatureAs line with a constant "--without-gcc-arch" or "--with-gcc-arch=generic"; I'm not sure which is better.