selftests/bpf: add bpf-gcc support

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>