fix: make UNREACHABLE() macro safe on compilers lacking __builtin_unreachable

Add fallback empty UNREACHABLE() definition for GCC < 4.5 and non-MSVC compilers, and insert return/break statements after all UNREACHABLE() calls to prevent -Wreturn-type warnings when the macro expands to nothing. Also cast malloc result to char* in io.c to suppress C++ compilation warning.

src/module/io.c

@@ -212,7 +212,7 @@ static void allocCallback(uv_handle_t* handle, size_t suggestedSize,
                           uv_buf_t* buf)
 {
   // TODO: Handle allocation failure.
-  buf->base = malloc(suggestedSize);
+  buf->base = (char*)malloc(suggestedSize);
   buf->len = suggestedSize;
 }