El Capitan system: always include GCC compiler

[scheibelp]

And add preference for %cce when setting compiler=cce

Some low level dependencies won't build with %cce, so this adds a GCC compiler definition regardless of what the user requests. To avoid using GCC in other cases, this adds a preference for %cce.

The changes here do not guarantee using %cce: doing so would likely require tighter coupling between the system and experiment class (which is in the works).

[rfhaque]

@scheibelp Is is possible to add versions for cce (e.g. [email protected], [email protected] etc), cray-mpich and other dependencies in llnl-elcapitan/system.py

[scheibelp]

Is is possible to add versions for cce (e.g. [email protected], [email protected] etc), cray-mpich and other dependencies in

Yes, in some sense handling this is a separate concern, and I could do so if you like (preferably in another PR).

In more detail: as a direct change, you take the current code in llnl-elcapitan/system.py

        if compiler == "cce":
            compiler_cfg_path = self.next_adhoc_cfg()
            with open(compiler_cfg_path, "w") as f:
                f.write(self.rocm_cce_compiler_cfg(rocm, "16.0.0"))

and change "16.0.0" to "18.0.0". To support both, I think it would make sense to add a conditional variant like

    variant(cce_version, values=("16", "18"), when="compiler=cce")

And then similarly, you would only need to change the one line