perf: arm_spe: Add format option for discard mode

FEAT_SPEv1p2 (optional from Armv8.6) adds a discard mode that allows all
SPE data to be discarded rather than written to memory. Add a format
bit for this mode.

If the mode isn't supported, the format bit isn't published and attempts
to use it will result in -EOPNOTSUPP. Allocating an aux buffer is still
allowed even though it won't be written to so that old tools continue to
work, but updated tools can choose to skip this step.

Signed-off-by: James Clark <james.clark@linaro.org>
Reviewd-by: Yeoreum Yun <yeoreum.yun@arm.com>
Link: https://lore.kernel.org/r/20250108142904.401139-2-james.clark@linaro.org


Signed-off-by: Will Deacon <will@kernel.org>