Issue #8559: improve unicode support of (gdb) libpython.py

* Escape non printable characters (use locale.getpreferredencoding())
 * Fix support of surrogate pairs
 * test_gdb.py: use ascii() instead of repr() in gdb program arguments to avoid
   encoding issues
 * Fix test_strings() of test_gdb.py for encoding different than UTF-8
   (eg. ACSII)