libc/minimal: locate the memory pool for malloc() to .bss

When CONFIG_USERSPACE is turned off, the POOL_SECTION will be located in
.data section.  This will increase the target binary size.  Since the
memory pool is for malloc() use and it doesn't need for initial values,
locate it in the .bss section to reduce binary size.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Change-Id: Iee52ac06a48414c083518c79775fe31334eab674