[WebAssembly] Rename target-features.ll (#96716)

I'm planning on a PR that splits `target-features.ll` into two different
files and fix some other stuff on them:
- `target-features-attrs.ll` that tests target features by bitcode
function attributes and `-mattr=` options
- `target-features-cpus.ll` that tests target features by `-mcpu=`
options

But `target-features-attrs.ll` will share a bulk of the lines with the
current `target-features.ll`. And if I remove `target-features.ll` and
create the two new files in a single PR, git doesn't recognize either of
them as a copy (I hoped at least `target-features-attrs.ll` would be
recognized as a copy because it shares many lines with the current file)

So to make the diff smaller and easier to review, I'm renaming the file
first. I'll follow up with the PR that does the actual splitting.

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