linker: sort app shared mem partition by alignment

If CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT is enabled,
the app shared memory partition may cause waste of memory
due to the need for padding.

For example, tests/subsys/jwt and board mps2_an385:

  z_test_mem_partition: addr 0x20000000, size 52
  z_libc_partition    : addr 0x20000040, size 4
  k_mbedtls_partition : addr 0x20008000, size 32736

    ending at 0x2000ffff, taking up 65536 bytes

With power-of-two size and alignment requirement,
k_mbedtls_partition takes up 32KB memory and needs to be
aligned on 32KB boundary. If the above partitions are
ordered as shown, there needs to be a lot of padding
after z_libc_partition before k_mbedtls_partition can
start. In order to minimize padding, these partitions
need to be sort by size in descending order.

After the changes here,	the partitions are:

  k_mbedtls_partition : addr 0x20000000, size 32736
  z_test_mem_partition: addr 0x20008000, size 52
  z_libc_partition    : addr 0x20008040, size 4

    ending at 0x2000805f, taking up 32864 bytes

With the above example, sorting results in a saving
of 32672 bytes of saving.

Fixes #14121

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