top(1): use greater warnings

One of the downsides of using numeric WARNS is that if we only have a
single type of issue we get no protection from other changes. For
example, we got no warning for missing variable declaration, due to
the issues with "const".

For this utility, explicitly list out the warnings which are failing.
They should still be fixed, so only reduce them to warning instead of
error.

Tested with: clang base (amd64, i386), gcc6, gcc7, gcc9, gcc base (mips)