impossible_compare: separate the impossible comparison check into one file

impossible_compare: separate the impossible comparison check into one file

The check_signed.c check was one of the first checks which I wrote and in
those days I used to combine all kinds of checks into one file.  These
days, I prefer to have one warning message per file because it's more
readable and easier to maintain.  So pull this into a separate file.

The other thing that this does is if it's an unsigned_var < 0 warning,
then do not print that.  It's already covered by check_unsigned_lt_zero.c.

And another thing that it does is it silences a bunch of false positives
in the kernel by making a list of allowed macros.  It's kind of a blunt
way to do it, but it works and is simple to implement.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>