Remove thread local attribute of prev_sig_act_SIGSEGV/SIGBUS to allow using
custom signal handler from non-main thread since in a thread spawned by
embedder, embedder may be unable to call wasm_runtime_init_thread_env to
initialize them.
And fix the handling of prev_sig_act when its sa_handler is SIG_DFL, SIG_IGN,
or a user customized handler.