[broken] cmake: fix crash on CC without libc support

[trofi]

Without this change pkgsLLVM fails to build any packages
as compiler-rt fails early in cmake:

CMake Error at ...-cmake-3.24.0/share/cmake-3.24/Modules/Platform/UnixPaths.cmake:53 (file):
  file STRINGS file
  "...-x86_64-unknown-linux-gnu-clang-wrapper-11.1.0/nix-support/orig-libc-dev"
  cannot be read.

It's a regression caused by 871cf9f "cmake: detect libc location
at runtime #181431" where we started using orig-libc-dev as a libc pointer.

During pkgsLLVM pootstrap first compiler has no libc support yet.

The change skips runtime detection if there are no libc signs.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • 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.

[trofi]

Almost the whole world rebuild:

$ ./maintainers/scripts/rebuild-amount.sh HEAD^
Estimating rebuild amount by counting changed Hydra jobs (parallel=unset).
      1 pkgs-lib-tests
  31506 x86_64-darwin
  31838 x86_64-linux

I'll re-target staging.

[trofi]

Confused github by pushing wrong branch /o\ into override. Trying again as #187777