imagemagick: Allow compilation without --with-gcc-arch

[zhaofengli]

Motivation for this change

The --with-gcc-arch flag is used to specify the value used for --march and --mtune [1], therefore it does not make sense to fail the evaluation simply because it's not enumerated by us. However, it's necessary to specify --without-gcc-arch, otherwise the config script will try to guess the values which leads to incompatible/non-determinstic artifacts.

I also have doubts whether some of the current values in the if arms are even correct. For example, aarch64 is not a valid value for -march/-mtune in aarch64 GCC. They "work" because the config script gracefully falls back (search for "whether C compiler accepts").

Tested by compiling imagemagick7 natively on riscv64 (with #115406 and #125132 for dependencies). A dependency of imagemagick6 does not build on riscv64 but that's a separate issue. This should not affect currently "supported" targets.

[1] https://github.com/ImageMagick/ImageMagick/blob/850d5a9fc5d8d6ae7534421737a1709efe164a77/m4/ax_gcc_archflag.m4

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/)
• Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.