lib: libc: common: Add malloc heap stats support

Add support for retrieving heap stats for the malloc heap in the common
libc malloc implementation. This provides the ability to see free and
used bytes using the same structure as the kernel heap.
Without this there is no method to retrieve the malloc heap usage as the
relevant sys_heap structure is private.

Signed-off-by: Graham Roff <grahamr@qti.qualcomm.com>