Rollup merge of #155656 - durin42:llvm-23-back-in-kansas, r=nikic

rustc_llvm: update opt-level handling for LLVM 23

LLVM 23 removed Os and Oz optimization pipelines and the PR says to use O2 with optsize or minsize instead as appropriate.

See https://github.com/llvm/llvm-project/pull/191363 for more details.