libc/common: Handle MMU/MPU/USERSPACE and alignment requirements in malloc

When CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE is set to -1, make the malloc
arena use all remaining RAM. When an MMU exists, allocate the arena at
startup using k_mem_map. Otherwise, compute the available memory
automatically and use that.

When an MPU is being used to manage the malloc arena, make sure the heap
respects any MPU alignment requirements. Otherwise, align the heap to
sizeof(double).

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