toolchain: migrate iterable sections calls to the external API

This migrates all the current iterable section usages to the external
API, dropping the "Z_" prefix:

Z_ITERABLE_SECTION_ROM
Z_ITERABLE_SECTION_ROM_GC_ALLOWED
Z_ITERABLE_SECTION_RAM
Z_ITERABLE_SECTION_RAM_GC_ALLOWED
Z_STRUCT_SECTION_ITERABLE
Z_STRUCT_SECTION_ITERABLE_ALTERNATE
Z_STRUCT_SECTION_FOREACH

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>