arm64: Enable FEAT_E0PD when supported

[andrew]

Summary

FEAT_E0PD adds two fields to the tcr_el1 special register that, when
set, cause userspace access to either the top or bottom half of the
address spaces without a page walk.

This can be used to stop userspace probing the kernel address space
as the CPU will raise an exception in the same time if the probed
address is in the TLB or not.

Sponsored by: Arm Ltd

[kevans]

sys/arm64/arm64/identcpu.c

2238

I note this will only get called on the boot AP since SI_SUB_CPU is pretty early, was that intentional or should it be run after other APs are running regularly @ SI_SUB_SMP?

[andrew]

Move later in the boot

[kevans]

Seems reasonable