otPlatCAlloc() function

Dynamically allocates new memory. On platforms that support it, should just redirect to calloc. For those that don't support calloc, should support the same functionality: "The calloc() function contiguously allocates enough space for count objects that are size bytes of memory each and returns a pointer to the allocated memory. The allocated memory is filled with bytes of value zero." Is required for OPENTHREAD_CONFIG_HEAP_EXTERNAL_ENABLE.