Feature request: global init block

[maleadt]

The init_block argument to build_tarballs currently generates code that's executed when a specific wrapper is found. That's generally fine, but in some cases we might also want to perform an action when no wrapper is available (say, perform system discovery, or log a message). I'd therefore like to propose to add a global_init_block argument to build_tarballs, and adapt JLLWrappers to call the wrappers' init functions from a global __init__ instead.

Thoughts?

[giordano]

I'm generally ok with adding more placeholders for greater flexibility.