ARM: ensure that processor vtables is not lost after boot

Marek Szyprowski reported problems with CPU hotplug in current kernels.
This was tracked down to the processor vtables being located in an
init section, and therefore discarded after kernel boot, despite being
required after boot to properly initialise the non-boot CPUs.

Arrange for these tables to end up in .rodata when required.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Fixes: 383fb3ee ("ARM: spectre-v2: per-CPU vtables to work around big.Little systems")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>