libc: add hidden option CONFIG_NEED_LIBC_MEM_PARTITION

The z_libc_partition was only enabled when newlib is being used,
and/or stack canaries are needed. This adds a hidden option
where this partition can be enabled if needed, regardless of
whether newlib is used or stack canaries are needed.

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