[llvm] [llvm-mca] Abort on parse error without -skip-unsupported-instructions (PR #90474)

[Andrea Di Biagio via llvm-commits]

adibiagio wrote:

Could you please add a brief description of flag `-skip-unsupported-instructions` to the llvm-mca command guide?

https://github.com/llvm/llvm-project/pull/90474