[compiler-rt] Check codesign in path before using (#99837)
Fixes an issue discovered with Nix where codesign is executed but Nix
cannot use codesign without faults. Use `find_program` in CMake to
ensure it is in `$PATH`.
Can be tested on macOS and change your path in such a way that
`codesign` isn't in it.
Related: https://github.com/NixOS/nixpkgs/pull/329058
