sys_sem: straighten build time definition macros

Commit db48d3e2 ("sys_sem: add build time definition macros")
recently introduced SYS_SEM_DEFINE() and defined it in terms of
Z_DECL_ALIGN() and __in_section() to force the _k_sem linker section.

It is however cleaner and less obscur to use Z_STRUCT_SECTION_ITERABLE()
and list the _sys_sem linker section alongside the _k_sem one.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>