selftests/bpf: List newest Clang built-ins needed for some CO-RE selftests

  https://reviews.llvm.org/D78466

has been pushed to llvm 10.x release branch and will be

available in 10.0.1. The fix is available in llvm 11.0.0 trunk.

BPF CO-RE-based tests and Clang version

=======================================

A set of selftests use BPF target-specific built-ins, which might require

bleeding-edge Clang versions (Clang 12 nightly at this time).

Few sub-tests of core_reloc test suit (part of test_progs test runner) require

the following built-ins, listed with corresponding Clang diffs introducing

them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too

old to support them, they shouldn't cause build failures or runtime test

failures:

  - __builtin_btf_type_id() ([0], [1], [2]);

  - __builtin_preserve_type_info(), __builtin_preserve_enum_value() ([3], [4]).

  [0] https://reviews.llvm.org/D74572

  [1] https://reviews.llvm.org/D74668

  [2] https://reviews.llvm.org/D85174

  [3] https://reviews.llvm.org/D83878

  [4] https://reviews.llvm.org/D83242

{

	/* We use __builtin_btf_type_id() in this tests, but up until the time

	 * __builtin_preserve_type_info() was added it contained a bug that

	 * would make this test fail. The bug was fixed with addition of

	 * would make this test fail. The bug was fixed ([0]) with addition of

	 * __builtin_preserve_type_info(), though, so that's what we are using

	 * to detect whether this test has to be executed, however strange

	 * that might look like.

	 *

	 *   [0] https://reviews.llvm.org/D85174

	 */

#if __has_builtin(__builtin_preserve_type_info)

	struct core_reloc_type_id_output *out = (void *)&data.out;