ldscripts: Place .preinit_array, .init_array and .fini_array in ROM.

.preinit_array will not work unless it is placed in ROM since it is
called before the C initialization which copies the .data section to
RAM. .init_array and .fini_array contains pointers to static constructors
and destructors, at the moment I do not see a use case where they need to
be writable at runtime.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>