[llvm] [NFC] add LLVM_ABI to function getMemcmp declaration (PR #166192)

[Lei Huang via llvm-commits]

https://github.com/lei137 approved this pull request.

this LGTM but I think you forgot about the doc requested via: https://github.com/llvm/llvm-project/pull/153600#discussion_r2363187904

https://github.com/llvm/llvm-project/pull/166192