swiftc: pass -Xcc before -march

[Samasaur1]

Description of changes

Adds the -Xcc flag before the -march flag passed to swiftc.

#291901 changed how clang got the -march flag, which caused swiftc to get that flag as well. However, swiftc needs -Xcc before -march because it's a clang flag, not a swift flag.

We could also simply prevent the -march flag from being passed to swiftc. I'm not familiar with exactly what this flag does, so I'm choosing to leave it in. However, note that #291901 seems to only have intended to add this flag to clang and accidentally added it to Swift, so maybe we do just want to remove it.

I know adding -Xcc made Swift able to build one of my Swift packages on aarch64-darwin, and I assume removing the flag will as well.

Closes #295322.
May supercede #296082.

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/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 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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[alyssais]

Haven't reviewed in depth because I don't know about swift, but I like the approach. -march tells the compiler to optimise for a specific architecture, so we probably want it for Swift.

[ghost]

@ofborg build swift

[ghost]

x64/aarch64 darwin builds pass on community builder (ofborg timesout)