gcc --coverage should be handled

When using gcc with the --coverage option a gcno file is created. By default the gcc has the option -o /dev/null in ale which leads to gcc trying to create /dev/null.gcno. This is usually not possible and therefore creates an error that is reported by ale.
I locally tried to replace it with ale#util#Tempname() and this seems to work. But I am unsure about the consequences of that change.