kernel: each mem slab buffer has its own section attribute

This attaches a unique section attribute for each mem slab
buffer defined with K_MEM_SLAB_DEFINE(). This allows them
to be placed as needed via linker scripts. This is useful
for demand paging as developers can choose which memory
slab buffer is pinned in memory.

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