veristat: add ability to sort by stat's absolute value

veristat: add ability to sort by stat's absolute value

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.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>