[Codegen][Tuner] Add support for default tuning specs (#19394)

These default specs are target architecture-specific and will be shipped
with the compiler.

* Default specs belong to target plugins and get embedded in
`libIREECompiler.so`, just like ukernels.
* Plugins then register their default tuning specs with the default
embedded directory.
* We store them as mlir text. We can't easily assemble them as mlir
bytecode without taking a circular dependency on iree-opt. We can
revisit this in the future and add a new tool `iree-as` that will only
link with dialects.
* After the initial loading, we cache the default specs in the IREE
codegen dialect transform library manager.
* Add a placeholder spec for gfx942.
* Document and test the inclusion order. User specs come before default
specs.

Issue: https://github.com/iree-org/iree/issues/19214