libc/picolibc: Place malloc heap in noinit section if possible

When the heap is of a fixed size and there isn't a special malloc partition
in use, place the heap in uninitialized memory so that the application
doesn't spend time at startup erasing it. Picolibc malloc always clears
memory before returning it to applications, so this change will not be
visible to applications.

Signed-off-by: Keith Packard <keithp@keithp.com>