add CI comments related to commented-out LLVM version entries in test matrix

[brody4hire]

coming from this comment: #195 (comment)

These are heavy to always be enabled, but they will be temporarily uncommented to see if they are affected by the LLVM bug if the code for this target is changed.

If we need some more info in these comments, I would appreciate any pointers to what LLVM issue(s) this is related to.

[brody4hire]

I think the build failure with Rust nightly & powerpc64-ibm-aix should go away once Rust nightly includes this rollup PR: rust-lang/rust#133059 ... which includes this: rust-lang/rust#133050

[taiki-e]

Sorry, but I don't think it is helpful to put these not very clear descriptions for each commented out code.

I this it is better to add matrix: level comments and state why some versions are commented out.

[brody4hire]

Rewritten & rebased please let me know if anything else is needed thanks

[taiki-e]

Thanks.