gen_app_partitions: add .sdata/.sbss section into app_smem

Some architectures (e.g. RISC-V) has .sdata/.sbss section for small
data/bss. Memory partition should also manage the permission of these
sections in library so they should be put into app_smem.
(For example, newlib _impure_ptr is in .sdata section and
__malloc_top_pad is in .sbss section in RISC-V.)

Signed-off-by: Jim Shu <cwshu@andestech.com>