arm64: trap implementation defined functionality in userspace

The Arm v8.8 extension adds a new control FEAT_TIDCP1 that allows the
kernel to disable all implementation-defined system registers and
instructions in userspace. This can improve robustness against covert
channels between processes, for example in cases where the firmware or
hardware didn't disable that functionality by default.

The kernel does not currently support any implementation-defined
features, as there are no hwcaps for any such features, so disable all
imp-def features unconditionally. Any use of imp-def instructions will
result in a SIGILL being delivered to the process (same as for undefined
instructions).

Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Link: https://lore.kernel.org/r/20220622115424.683520-1-kristina.martsenko@arm.com
Signed-off-by: Will Deacon <will@kernel.org>