Merge branch 'type-and-enum-value-relos'

Andrii Nakryiko says:

====================
This patch set adds libbpf support for two new classes of CO-RE relocations:
type-based (TYPE_EXISTS/TYPE_SIZE/TYPE_ID_LOCAL/TYPE_ID_TARGET) and enum
value-vased (ENUMVAL_EXISTS/ENUMVAL_VALUE):
  - TYPE_EXISTS allows to detect presence in kernel BTF of a locally-recorded
    BTF type. Useful for feature detection (new functionality often comes with
    new internal kernel types), as well as handling type renames and bigger
    refactorings.
  - TYPE_SIZE allows to get the real size (in bytes) of a specified kernel
    type. Useful for dumping internal structure as-is through perfbuf or
    ringbuf.
  - TYPE_ID_LOCAL/TYPE_ID_TARGET allow to capture BTF type ID of a BTF type in
    program's BTF or kernel BTF, respectively. These could be used for
    high-performance and space-efficient generic data dumping/logging by
    relying on small and cheap BTF type ID as a data layout descriptor, for
    post-processing on user-space side.
  - ENUMVAL_EXISTS can be used for detecting the presence of enumerator value
    in kernel's enum type. Most direct application is to detect BPF helper
    support in kernel.
  - ENUMVAL_VALUE allows to relocate real integer value of kernel enumerator
    value, which is subject to change (e.g., always a potential issue for
    internal, non-UAPI, kernel enums).

I've indicated potential applications for these relocations, but relocations
themselves are generic and unassuming and are designed to work correctly even
in unintended applications. Furthermore, relocated values become constants,
known to the verifier and could and would be used for dead branch code
detection and elimination. This makes them ideal to do all sorts of feature
detection and guarding functionality that's not available on some older (but
still supported by BPF program) kernels, while having to compile and maintain
one unified source code.

Selftests are added for all the new features. Selftests utilizing new Clang
built-ins are designed such that they will compile with older Clangs and will
be skipped during test runs. So this shouldn't cause any build and test
failures on systems with slightly outdated Clang compiler.

LLVM patches adding these relocation in Clang:
  - __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

v2->v3:
  - fix feature detection for __builtin_btf_type_id() test (Yonghong);
  - fix extra empty lines at the end of files (Yonghong);

v1->v2:
  - selftests detect built-in support and are skipped if not found (Alexei).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>