[llvm] [WebAssembly][NFC] Remove direct access to FeatureKV (PR #206232)

[Alexis Engelke via llvm-commits]

aengelke wrote:

[Entire planned change stack](https://github.com/aengelke/llvm-project/compare/49f787b8278d...featurekv) (5 commits).

In an all-target build, this change to FeatureKV will shrink .data.rel.ro by ~139 kiB, which currently need to be touched on every startup.

https://github.com/llvm/llvm-project/pull/206232