Add toolchain setup check for OpenCL

[andrjohns]

Summary

It was identified on the forums that OpenCL models can fail to compile without a helpful error message: https://discourse.mc-stan.org/t/cmdstan-installation-error/32900/5

I think we should add a configuration/setup step for OpenCL so we can catch the error and recommend running the setup function. I'm currently leaning towards adding it to check_cmdstan_toolchain, like:

check_cmdstan_toolchain(fix = TRUE, opencl = TRUE)

Thoughts/prefs?

[jgabry]

Yeah I think adding it to check_cmdstan_toolchain is a good idea

[rok-cesnovar]

Agreed, great idea!