malloc: Enable SYS_MALLOC_RUNTIME_INIT by default in SPL

On boards with size restrictions, 1-2k can be a significant fraction of
the binary size. Add a new SPL version of SYS_MALLOC_RUNTIME_INIT and
enable it by default.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>