STMs: Microlib heap fix for new, malloc and friends

heap can grow outside 0x400 default size and does not overlap stack area.
I checked on F401 board, the only one i've actually, but isue should be reflected to all other stm32 whith arm micro toolchain.
I really don't know about rtos, i never used so far, i just looked at src and seems it uses new for creating thread stacks, so probably it could fix something there also....or break it:)