heap-management: Use C memory management APIs

We shouldn't be allocating heap using C and FreeRTOS
heap management APIs as we might end up with both FreeRTOS heap
and C heap. This can lead to several unexpected issues.

C standard heap functions are used to unify heap allocations
as we can't use FreeRTOS heap management APIs because this
would require patching lots of file as part of
ml-embedded-evaluation-kit library which use C standard heap
functions for heap allocations.

Heap space is increased by `0x10000` as C standard heap
management APIs requires more heap space for the existing
FRI applications because it is less optimised than FreeRTOS heap
management APIS (heap_4).

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>