Only add alignment padding when needed (#650)

Only add alignment padding when needed (#650)

Heap 4 and Heap 5 add some padding to ensure that the allocated blocks
are always aligned to portBYTE_ALIGNMENT bytes. The code until now was
adding padding always even if the resulting block was already aligned.
This commits updates the code to only add padding if the resulting block
is not aligned.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>