arm64: Add HWCAP advertising FEAT_WFXT

In order to allow userspace to enjoy WFET, add a new HWCAP that
advertises it when available.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220419182755.601427-9-maz@kernel.org