Add ability to sort results by absolute values of specified stats. This
is especially useful to find biggest deviations in comparison mode. When
comparing verifier change effect against a large base of BPF object
files, it's necessary to see big changes both in positive and negative
directions, as both might be a signal for regressions or bugs.

The syntax is natural, e.g., adding `-s '|insns_diff|'^` will instruct
veristat to sort by absolute value of instructions difference in
ascending order.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231108051430.1830950-1-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>