selftests/bpf: Add BTF_KIND_FLOAT to test_core_reloc_size

Verify that bpf_core_field_size() is working correctly with floats.
Also document the required clang version.

Suggested-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210309005649.162480-2-iii@linux.ibm.com