arm64/sve: Kconfig update and conditional compilation support
This patch adds CONFIG_ARM64_SVE to control building of SVE support
into the kernel, and adds a stub predicate system_supports_sve() to
control conditional compilation and runtime SVE support.
system_supports_sve() just returns false for now: it will be
replaced with a non-trivial implementation in a later patch, once
SVE support is complete enough to be enabled safely.