libc/minimal: fix realloc() allocated memory alignment

The definition for realloc() says that it should return a pointer
to the allocated memory which is suitably aligned for any built-in
type.

Turn sys_heap_realloc() into a sys_heap_aligned_realloc() and use it
with __alignof__(z_max_align_t) to implement realloc() with proper
memory alignment for any platform.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>