Avoid gcov filenames too long

[zhongpan]

In PR add support for Ninja #22, I realized that I had introduced two issues:

1. The gcov file name may exceed the maximum length limit
2. When build gcov target, always regenerate the gcov file, because the OUTPUT parameter in the add_custom_command command is not the actual generated file.

Solutions:

1. Replace -p with -x, and move the gcov file to the path corresponding to the hash(-x) for better readability
2. OUTPUT is the file moved above

[alehaa]

Can you link some resources about the "maximum length limit"? As fas as I know a path may take up to 4096 characters in Linux.