[WebAssembly] Update supported features in the generic CPU configuration

Enable sign-ext and mutable-globals in -mcpu=generic. This makes these features
enabled by default.

These features are all [finished proposals], and all major wasm engines
support them.

[finished proposals]: https://github.com/WebAssembly/proposals/blob/main/finished-proposals.md

Differential Revision: https://reviews.llvm.org/D125728