AF_XDP: add xsk.{c,h} to Makefile and fix build

This patch makes sure we build AF_XDP-related code as part of libbpf. This
also required copying few uapi/linux headers and adding few used definitions
in include headers.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>

include/linux/compiler.h

@@ -0,0 +1,52 @@
+/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
+
+#ifndef __LINUX_COMPILER_H
+#define __LINUX_COMPILER_H
+
+#define likely(x)		__builtin_expect(!!(x), 1)
+#define unlikely(x)		__builtin_expect(!!(x), 0)
+
+#define READ_ONCE(x)		(*(volatile typeof(x) *)&x)
+#define WRITE_ONCE(x, v)	(*(volatile typeof(x) *)&x) = (v)
+
+#define barrier()		asm volatile("" ::: "memory")
+
+#if defined(__x86_64__)
+
+# define smp_rmb()		asm volatile("lfence" ::: "memory")
+# define smp_wmb()		asm volatile("sfence" ::: "memory")
+
+# define smp_store_release(p, v)		\
+do {						\
+	barrier();				\
+	WRITE_ONCE(*p, v);			\
+} while (0)
+
+# define smp_load_acquire(p)			\
+({						\
+	typeof(*p) ___p = READ_ONCE(*p);	\
+	barrier();				\
+	___p;					\
+})
+
+#else
+
+# define smp_mb()		__sync_synchronize()
+# define smp_rmb()		smp_mb()
+# define smp_wmb()		smp_mb()
+
+# define smp_store_release(p, v)		\
+do {						\
+	smp_mb();				\
+	WRITE_ONCE(*p, v);			\
+} while (0)
+
+# define smp_load_acquire(p)			\
+({						\
+	typeof(*p) ___p = READ_ONCE(*p);	\
+	smp_mb();				\
+	___p;					\
+})
+
+#endif /* defined(__x86_64__) */
+#endif