libc/common: Allocate default malloc arena for non-minimal-libc uses

When using the common malloc implementation on systems not using the
minimal C library, allocate a reasonable default malloc heap according to
the following rules (adopted from the Picolibc heap size defaults):

* When an MMU is available, allocate 16kB.
* When USERSPACE is enabled for a device with an MPU require PoT alignment,
  allocate 1024 bytes
* Otherwise, use all available memory.

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