arm64/sve: Add sysctl to set the default vector length for new processes

Because of the effect of SVE on the size of the signal frame, the
default vector length used for new processes involves a tradeoff
between performance of SVE-enabled software on the one hand, and
reliability of non-SVE-aware software on the other hand.

For this reason, the best choice depends on the repertoire of
userspace software in use and is thus best left up to distro
maintainers, sysadmins and developers.

If CONFIG_SYSCTL is enabled, this patch exposes the default vector
length in /proc/sys/abi/sve_default_vector_length, where boot
scripts or the adventurous can poke it.

In common with other arm64 ABI sysctls, this control is currently
global: setting it requires CAP_SYS_ADMIN in the root user
namespace, but the value set is effective for subsequent execs in
all namespaces.  The control only affects _new_ processes, however:
changing it does not affect the vector length of any existing
process.

The intended usage model is that if userspace is known to be fully
SVE-tolerant (or a developer is curious to find out) then init
scripts can crank this up during startup.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>