This commit adds a dynamic memory tracer that calls a callback whenever
one of the basic memory allocation functions (malloc, realloc, calloc,
free) is called. The operation of the tracer is guarded by the
'MBED_MEM_TRACING_ENABLED` macro.
Infinite recursion during tracing is guarded by using atomic
increment/decrement primitives on the `trace_level` variable.
Thanks to @c1728p9 and @heky for their help and suggestions.
Sam Grove
Martin Kojtal
Neil Thiessen
Bogdan Marinescu