mempool: fully use the inline free block bitmap on 64-bit targets

The "bits" field in struct sys_mem_pool_lvl is unioned with a pointer.
That leaves more space for inline free bits on 64-bit targets.
Let's declare it as an array and adjust its size based on the pointer
size. On 32-bit targets the generated code remains identical.

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