arm64: fix undefined reference to 'printk'

The printk symbol was intended as a generic address that is always
exported, however that turned out to be false with CONFIG_PRINTK=n:

ERROR: "printk" [arch/arm64/kernel/arm64-reloc-test.ko] undefined!

This changes the references to memstart_addr, which should be there
regardless of configuration.

Fixes: a257e025

 ("arm64/kernel: don't ban ADRP to work around Cortex-A53 erratum #843419")
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>