[llvm] [RISCV] Don't use std::vector<std::string> for split extensions in RISCVISAInfo::parseArchString. NFC (PR #91538)

[Jessica Clarke via llvm-commits]

https://github.com/jrtc27 approved this pull request.

Makes sense to me. I have an idea I might try to shave off the empty special casing before splitting, but it could end up uglier in the end, so assume that to be the case if you hear nothing further :)

https://github.com/llvm/llvm-project/pull/91538