Add BTI landing pads to the AArch64 SHA2 assembly

The Arm Branch Target Identification (BTI) extension guards against
branching to an unintended instruction.

To support BTI add the landing pad instructions to the SHA2 functions.
These are from the hint space so are a nop on hardware that lacks BTI
support or if BTI isn't enabled.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Andrew Turner <andrew.turner4@arm.com>
Closes #14862
Closes #15339