[libc++/abi/unwind] Rename Lit features for no exceptions to 'no-exceptions'

Instead of having different names for the same Lit feature accross code
bases, use the same name everywhere. This NFC commit is in preparation
for a refactor where all three projects will be using the same Lit
feature detection logic, and hence it won't be convenient to use
different names for the feature.

Differential Revision: https://reviews.llvm.org/D78370