selftests/bpf: Fix a clang deprecated-declarations compilation error

Build the kernel and selftest with clang compiler with LLVM=1,
  make -j LLVM=1
  make -C tools/testing/selftests/bpf -j LLVM=1

I hit the following selftests/bpf compilation error:
  In file included from test_cpp.cpp:3:
  /.../tools/testing/selftests/bpf/tools/include/bpf/libbpf.h:73:8:
    error: 'relaxed_core_relocs' is deprecated: libbpf v0.6+: field has no effect [-Werror,-Wdeprecated-declarations]
  struct bpf_object_open_opts {
         ^
  test_cpp.cpp:56:2: note: in implicit move constructor for 'bpf_object_open_opts' first required here
          LIBBPF_OPTS(bpf_object_open_opts, opts);
          ^
  /.../tools/testing/selftests/bpf/tools/include/bpf/libbpf_common.h:77:3: note: expanded from macro 'LIBBPF_OPTS'
                  (struct TYPE) {                                             \
                  ^
  /.../tools/testing/selftests/bpf/tools/include/bpf/libbpf.h:90:2: note: 'relaxed_core_relocs' has been explicitly marked deprecated here
          LIBBPF_DEPRECATED_SINCE(0, 6, "field has no effect")
          ^
  /.../tools/testing/selftests/bpf/tools/include/bpf/libbpf_common.h:24:4: note: expanded from macro 'LIBBPF_DEPRECATED_SINCE'
                  (LIBBPF_DEPRECATED("libbpf v" # major "." # minor "+: " msg))
                   ^
  /.../tools/testing/selftests/bpf/tools/include/bpf/libbpf_common.h:19:47: note: expanded from macro 'LIBBPF_DEPRECATED'
  #define LIBBPF_DEPRECATED(msg) __attribute__((deprecated(msg)))

There are two ways to fix the issue, one is to use GCC diagnostic ignore pragma, and the
other is to open code bpf_object_open_opts instead of using LIBBPF_OPTS.
Since in general LIBBPF_OPTS is preferred, the patch fixed the issue by
adding proper GCC diagnostic ignore pragmas.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220217194005.2765348-1-yhs@fb.com

tools/testing/selftests/bpf/test_cpp.cpp

 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
 #include <iostream>
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 #include <bpf/libbpf.h>
+#pragma GCC diagnostic pop
 #include <bpf/bpf.h>
 #include <bpf/btf.h>
 #include "test_core_extern.skel.h"