arch_init: Fix format string by using RAM_ADDR_FMT

length is a ram_addr_t, so RAM_ADDR_FMT must be used instead of %ld.
This fixes a recently introduced regression for w64 builds.

Using RAM_ADDR_FMT also changes decimal output to sedecimal.
This is good here because length and block->length should both
use the same base in the error message.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Alexander Graf <agraf@suse.de>