selftests/bpf: Make selftest compilation work on clang 11

We can't compile test_core_reloc_module.c selftest with clang 11, compile
fails with:

  CLNG-LLC [test_maps] test_core_reloc_module.o
  progs/test_core_reloc_module.c:57:21: error: use of unknown builtin \
  '__builtin_preserve_type_info' [-Wimplicit-function-declaration]
   out->read_ctx_sz = bpf_core_type_size(struct bpf_testmod_test_read_ctx);

Skipping these tests if __builtin_preserve_type_info() is not supported
by compiler.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201209142912.99145-1-jolsa@kernel.org