Follow-up on #2907. The log level is needed in the host embedder to
better integrate with the embedder's logger.
Allow the developer to customize his bh_log callback with
`cmake -DWAMR_BH_LOG=<log_callback>`,
and update sample/basic to show the usage.
In some platforms, allocating memory with size 0 may return NULL but not an empty memory block, which causes runtime load, instantiate or execute wasm/aot file failed. We add checks to try to avoid allocating memory in runtime if the size is 0. And in wasm_runtime_malloc/free, output warning if allocate memory with size 0 and free memory with NULL ptr.
Also fix some coding style issues, fix handle riscv32 ilp32d issue, and fix several wasm-c-api issues.
Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>
