Comment 3 for bug 1879360

Build log on jenkins is complaining about the use of unknown builtin '__builtin_preserve_field_info:

    ^
    progs/test_core_reloc_bitfields_direct.c:59:13: error: use of unknown builtin '__builtin_preserve_field_info' [-Wimplicit-function-declaration]
    out->s32 = BPF_CORE_READ_BITFIELD(in, s32);
    ^
    /home/ubuntu/autotest/client/tmp/ubuntu_bpf/src/linux/tools/testing/selftests/bpf/tools/include/bpf/bpf_core_read.h:68:36: note: expanded from macro 'BPF_CORE_READ_BITFIELD'
    const void *p = (const void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
    ^
    /home/ubuntu/autotest/client/tmp/ubuntu_bpf/src/linux/tools/testing/selftests/bpf/tools/include/bpf/bpf_core_read.h:23:2: note: expanded from macro '__CORE_RELO'
    __builtin_preserve_field_info((src)->field, BPF_FIELD_##info)
    ^
    7 errors generated.
    llc-9: error: llc-9: :1:1: error: expected top-level entity
    BPF obj compilation failed
    ^