selftests/bpf: add bpf-gcc support

Now that binutils and gcc support for BPF is upstream, make use of it in
BPF selftests using alu32-like approach. Share as much as possible of
CFLAGS calculation with clang.

Fixes only obvious issues, leaving more complex ones for later:
- Use gcc-provided bpf-helpers.h instead of manually defining the
  helpers, change bpf_helpers.h include guard to avoid conflict.
- Include <linux/stddef.h> for __always_inline.
- Add $(OUTPUT)/../usr/include to include path in order to use local
  kernel headers instead of system kernel headers when building with O=.

In order to activate the bpf-gcc support, one needs to configure
binutils and gcc with --target=bpf and make them available in $PATH. In
particular, gcc must be installed as `bpf-gcc`, which is the default.

Right now with binutils 25a2915e8dba and gcc r275589 only a handful of
tests work:

	# ./test_progs_bpf_gcc
	# Summary: 7/39 PASSED, 1 SKIPPED, 98 FAILED

The reason for those failures are as follows:

- Build errors:
  - `error: too many function arguments for eBPF` for __always_inline
    functions read_str_var and read_map_var - must be inlining issue,
    and for process_l3_headers_v6, which relies on optimizing away
    function arguments.
  - `error: indirect call in function, which are not supported by eBPF`
    where there are no obvious indirect calls in the source calls, e.g.
    in __encap_ipip_none.
  - `error: field 'lock' has incomplete type` for fields of `struct
    bpf_spin_lock` type - bpf_spin_lock is re#defined by bpf-helpers.h,
    so its usage is sensitive to order of #includes.
  - `error: eBPF stack limit exceeded` in sysctl_tcp_mem.
- Load errors:
  - Missing object files due to above build errors.
  - `libbpf: failed to create map (name: 'test_ver.bss')`.
  - `libbpf: object file doesn't contain bpf program`.
  - `libbpf: Program '.text' contains unrecognized relo data pointing to
    section 0`.
  - `libbpf: BTF is required, but is missing or corrupted` - no BTF
    support in gcc yet.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/bpf_helpers.h

@@ -1,12 +1,6 @@
 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
-#ifndef __BPF_HELPERS_H
-#define __BPF_HELPERS_H
-
-/* helper macro to place programs, maps, license in
- * different sections in elf_bpf file. Section names
- * are interpreted by elf_bpf loader
- */
-#define SEC(NAME) __attribute__((section(NAME), used))
+#ifndef __BPF_HELPERS__
+#define __BPF_HELPERS__
 
 #define __uint(name, val) int (*name)[val]
 #define __type(name, val) val *name
@@ -19,6 +13,14 @@
 			 ##__VA_ARGS__);		\
 })
 
+#ifdef __clang__
+
+/* helper macro to place programs, maps, license in
+ * different sections in elf_bpf file. Section names
+ * are interpreted by elf_bpf loader
+ */
+#define SEC(NAME) __attribute__((section(NAME), used))
+
 /* helper functions called from eBPF programs written in C */
 static void *(*bpf_map_lookup_elem)(void *map, const void *key) =
 	(void *) BPF_FUNC_map_lookup_elem;
@@ -256,6 +258,12 @@ struct bpf_map_def {
 	unsigned int numa_node;
 };
 
+#else
+
+#include <bpf-helpers.h>
+
+#endif
+
 #define BPF_ANNOTATE_KV_PAIR(name, type_key, type_val)		\
 	struct ____btf_map_##name {				\
 		type_key key;					\