add i32 support and make it extensible

tests/c-form/i32test.bpf.c

@@ -0,0 +1,15 @@
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("xdp")
+int print_xdp_data(struct xdp_md *ctx)
+{
+    // 'data' is a pointer to the start of packet data
+    void *data = (void *)(long)ctx->data;
+
+    bpf_printk("ctx->data = %p\n", data);
+
+    return XDP_PASS;
+}
+
+char LICENSE[] SEC("license") = "GPL";