output of `-C help` should be more helpful

[JeffBezanson]

We're relying on LLVM to print the list of supported CPUs and features, and as a result the output is confusing as it contains

For example, llc -mcpu=mycpu -mattr=+feature1,-feature2

For us I believe the syntax is -C cpuname,+feature1,-feature2;...;..., where ; delimits different configurations to support. Each configuration can also contain clone_all, in which case all code will be (re)generated for that architecture, or base(n) meaning we will try to share code with configuration #n (0-based I think?).

There is a comment about this in jl_init_llvm.

See #21849

[yuyichao]

The main doc is actually here. I assume sysimg building doc is in devdoc since, well, these are mainly for people that need to compile julia.

Adding more output to -C help is fine, though I'm pretty sure I've checked LLVM source and couldn't find a way to disable llc specific cmdline hint.

[JeffBezanson]

Yes, it would help a lot to at least add the julia syntax to the help text.