Remove gcov-generated files in root parrot dir with 'make cover-clean'

The *.gcov, *.gcda, *.gcno files, which are generated by gcov, are also
output in the base parrot directory.  'make cover-clean' didn't remove these
files.  This commit corrects this issue.
Removing cover-generated html files in 'make cover-clean'
Added frontend/pbc_disassemble to list of coverage directories
Merge pull request #949 from paultcochrane/ptc/cover_fixups

Clean up better after 'make cover'