arm64: Save state of HCR_EL2.E2H before switch to EL1

As we're about to switch the way E2H-stuck CPUs boot, save
the boot CPU E2H state as a flag tied to the boot mode
that can then be checked by the idreg override code.

This allows us to replace the is_kernel_in_hyp_mode() check
with a simple comparison with this state, even when running
at EL1. Note that this flag isn't saved in __boot_cpu_mode,
and is only kept in a register in the assembly code.

Use with caution.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220630160500.1536744-3-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>