toolchain: common: iterable sections: Add generic type macros

Add a set of macros that can be utilized for any type and redefine
the struct macros to utilize these macros.  While the majority of
uses for iterable sections are for structs, there are some cases
where the elements of the section might be a union or some other
type.

Also added <STRUCT|TYPE>_SECTION_<START|END>_EXTERN helper macros to
give a common means to handle setting externs in rare case they
are needed.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>