KVM: arm64: nv: Take "masked" aborts to EL2 when HCRX_EL2.TMEA is set

HCRX_EL2.TMEA further modifies the external abort behavior where
unmasked aborts are taken to EL1 and masked aborts are taken to EL2.
It's rather weird when you consider that SEAs are, well, *synchronous*
and therefore not actually maskable. However, for the purposes of
exception routing, they're considered "masked" if the A flag is set.

This gets a bit hairier when considering the fact that TMEA
also enables vSErrors, i.e. KVM has delegated the HW vSError context to
the guest hypervisor.  We can keep the vSError context delegation as-is
by taking advantage of a couple properties:

 - If SErrors are unmasked, the 'physical' SError can be taken
   in-context immediately. In other words, KVM can emulate the EL1
   SError while preserving vEL2's ownership of the vSError context.

 - If SErrors are masked, the 'physical' SError is taken to EL2
   immediately and needs the usual nested exception entry.

Note that the new in-context handling has the benign effect where
unmasked SError injections are emulated even for non-nested VMs.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-19-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

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