tools/bpftool: Add BTF_KIND_FLOAT support

Only dumping support needs to be adjusted, the code structure follows
that of BTF_KIND_INT. Example plain and JSON outputs:

    [4] FLOAT 'float' size=4
    {"id":4,"kind":"FLOAT","name":"float","size":4}

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210226202256.116518-5-iii@linux.ibm.com