selftests/bpf: Add GCC compatible builtins to bpf_legacy.h

The bpf_legacy.h header uses llvm specific load functions, add
GCC compatible variants as well to fix tests using these functions
under GCC.
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221201190939.3230513-1-james.hilliard1@gmail.com

tools/testing/selftests/bpf/bpf_legacy.h

@@ -2,15 +2,22 @@
 #ifndef __BPF_LEGACY__
 #define __BPF_LEGACY__
 
+#if __GNUC__ && !__clang__
+/* Functions to emit BPF_LD_ABS and BPF_LD_IND instructions.  We
+ * provide the "standard" names as synonyms of the corresponding GCC
+ * builtins. Note how the SKB argument is ignored.
+ */
+#define load_byte(skb, off) __builtin_bpf_load_byte(off)
+#define load_half(skb, off) __builtin_bpf_load_half(off)
+#define load_word(skb, off) __builtin_bpf_load_word(off)
+#else
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions
  */
-unsigned long long load_byte(void *skb,
-			     unsigned long long off) asm("llvm.bpf.load.byte");
-unsigned long long load_half(void *skb,
-			     unsigned long long off) asm("llvm.bpf.load.half");
-unsigned long long load_word(void *skb,
-			     unsigned long long off) asm("llvm.bpf.load.word");
+unsigned long long load_byte(void *skb, unsigned long long off) asm("llvm.bpf.load.byte");
+unsigned long long load_half(void *skb, unsigned long long off) asm("llvm.bpf.load.half");
+unsigned long long load_word(void *skb, unsigned long long off) asm("llvm.bpf.load.word");
+#endif
 
 #endif