[clang] [RISCV] Fix collectNonISAExtFeature returning negative extension features (PR #76962)

[Luke Lau via cfe-commits]

lukel97 wrote:

Note that this doesn't fix the issue described in https://github.com/llvm/llvm-project/pull/74889#pullrequestreview-1773445559. One approach that would build upon this would be to use the entire list of target features including negative extensions when a full arch string is specified.

https://github.com/llvm/llvm-project/pull/76962