libio: Convert __vasprintf_internal to buffers

The buffer resizing algorithm is slightly different.  The initial
buffer is on the stack, and small buffers are directly allocated
on the heap using the exact required size.  The overhead of the
additional copy is compensated by the lowered setup cost for buffers
compared to libio streams.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>