arm64: alternatives: add __init/__initconst to some functions/variables

apply_alternatives_vdso(), __apply_alternatives_multi_stop() and
kernel_alternatives are not needed after booting, so mark the two
functions as __init and the var as __initconst.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20221202161859.2228-1-jszhang@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>