arm64 updates for 5.2

Mostly just incremental improvements here:

- Introduce AT_HWCAP2 for advertising CPU features to userspace

- Expose SVE2 availability to userspace

- Support for "data cache clean to point of deep persistence" (DC PODP)

- Honour "mitigations=off" on the cmdline and advertise status via sysfs

- CPU timer erratum workaround (Neoverse-N1 #1188873)

- Introduce perf PMU driver for the SMMUv3 performance counters

- Add config option to disable the kuser helpers page for AArch32 tasks

- Futex modifications to ensure liveness under contention

- Rework debug exception handling to seperate kernel and user handlers

- Non-critical fixes and cleanup
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Merge branch 'for-next/perf' of git://git.kernel.org/pub/scm/linux/kernel/git/will/linux into for-next/core