Merge branch 'selftests/bpf: Add read_build_id function'

Jiri Olsa says:

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hi,
this selftests cleanup was previously posted as part of file build id changes [1],
which might take more time, so I'm sending the selftests changes separately so it
won't get stuck.

v4 changes:
  - added size argument to read_build_id [Andrii]
  - condition changes in parse_build_id_buf [Andrii]
  - use ELF_C_READ_MMAP in elf_begin [Andrii]
  - return -ENOENT in read_build_id if build id is not found [Andrii]
  - dropped elf class check [Andrii]

thanks,
jirka

[1] https://lore.kernel.org/bpf/20230316170149.4106586-1-jolsa@kernel.org/
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Signed-off-by: Alexei Starovoitov <ast@kernel.org>