tools/bpf: refactor test_btf pretty printing for multiple map value formats

The test_btf pretty print is refactored in order to easily
support multiple map value formats. The next patch will
add __int128 type tests which needs macro guard __SIZEOF_INT128__.
There is no functionality change with this patch.

Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>