Use -fprofile_dir=${COVERAGEDIR} with --coverage

This option implies that it will automatically prefix .gcda files, when
created, thus allowing for the desired prefixing I've added in via bsd.cov.mk .

This option is only available in clang and later versions of gcc, so I'll
likely have to mark MK_COVERAGE broken from a "fully-fledged feature"
perspective, as I don't want to go through the annoyance of implementing the
feature in gcc and testing it.