arm64: head: Run feature override detection before mapping the kernel

To permit the feature overrides to be taken into account before the
KASLR init code runs and the kernel mapping is created, move the
detection code to an earlier stage in the boot.

In a subsequent patch, this will be taken advantage of by merging the
preliminary and permanent mappings of the kernel text and data into a
single one that gets created and relocated before start_kernel() is
called.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-53-ardb+git@google.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>