selftests/bpf: allow to define asc/desc ordering for sort specs in veristat

selftests/bpf: allow to define asc/desc ordering for sort specs in veristat

Allow to specify '^' at the end of stat name to designate that it should
be sorted in ascending order. Similarly, allow any of 'v', 'V', '.',
'!', or '_' suffix "symbols" to designate descending order. It's such
a zoo for descending order because there is no single intuitive symbol
that could be used (using 'v' looks pretty weird in practice), so few
symbols that are "downwards leaning or pointing" were chosen. Either
way, it shouldn't cause any troubles in practice.

This new feature allows to customize sortering order to match user's
needs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221103055304.2904589-6-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>