Is there an equivalent to .spec files for Clang/LLVM and where can I find a reference?

According to the docs, the Clang driver doesn't have a direct equivalent to GCC spec files:

The clang driver has no direct correspondent for “specs”. The majority of the functionality that is embedded in specs is in the Tool specific argument translation routines. The parts of specs which control the compilation pipeline are generally part of the Pipeline stage.

The #include search path can be overridden using -nostdinc and -isystem.

However, I don't know how to override the linker or fully override the library search path.