KVM: arm64: Factor out helper for selecting exception target EL

Pull out the exception target selection from pend_sync_exception() for
general use. Use PSR_MODE_ELxh as a shorthand for the target EL, as
SP_ELx selection is handled further along in the hyp's exception
emulation.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-16-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>