New Linker script to add heap

Edited Atmel's linker script to add a heap.
_sbrk function added to startup_sam4s.c to check if memory is available
(needed for malloc).

In sam4s16c_flash.ld you can keep HEAP_SIZE commented to make the heap
size all the remaining space. If HEAP_SIZE is defined, then that is the
heap size. (see sam4s16c_flash.ld).