arm64: Add support for SB barrier and patch in over DSB; ISB sequences

[ Upstream commit bd4fb6d270bc423a9a4098108784f7f9254c4e6d ]

We currently use a DSB; ISB sequence to inhibit speculation in set_fs().
Whilst this works for current CPUs, future CPUs may implement a new SB
barrier instruction which acts as an architected speculation barrier.

On CPUs that support it, patch in an SB; NOP sequence over the DSB; ISB
sequence and advertise the presence of the new instruction to userspace.

Signed-off-by: Will Deacon <will.deacon@arm.com>
[ Mark: fixup conflicts ]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>