linker: add macros so iterable sections can be garbage collected

The iterable section macros Z_ITERABLE_SECTION_ROM()/_RAM() uses
Z_LINK_ITERABLE() which does KEEP() on all symbols. This results
in all symbols under those sections being kept in final image
even though these symbols are not referred directly from the code.
This adds the new set of macros which does not force KEEP() on
the symbols, and allows symbols to be garbage collected.
The existing macros are kept as-is so as not to change their
behaviors.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>