basic/log: do not treat all negative errnos as synthetic

basic/log: do not treat all negative errnos as synthetic

Currently, IS_SYNTHETIC_ERRNO() evaluates to true for all negative errnos,
because of the two's-complement negative value representation.
Subsequently, ERRNO= is not logged for most of our own code.
Let's fix this, by formatting all synthetic errnos as positive.
Then, treat all negative values as non-synthetic.

While at it, mark the evaluation order explicitly, and remove
unneeded comment.

Fixes #33800