Delete gcov files more selectively.

Previously, make target 'clean' would delete all '*.gc*' files, this would
erroneously delete 'src/polarssl/tests/suites/test_suite_cipher.gcm.data'.

This contributed to marking the src/polarssl submodule dirty and caused
polarssl's test suite to fail (as was noticed and worked around in issue #206).

With this patch, it deletes only '*.gcno' and '*.gcda' files.