refactor: Rename SVE-specific files

In preparation to the SME support patches, rename the SVE-specific files to
something a bit more meaningful that can be shared with the SME code.

In this case, I've renamed the "sve" in the names to "scalable".

No functional changes.

Regression-tested on aarch64-linux Ubuntu 22.04/20.04.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>