gdb/python: New InferiorThread.ptid_string attribute

gdb/python: New InferiorThread.ptid_string attribute

This commit adds a new InferiorThread.ptid_string attribute.  This
read-only attribute contains the string returned by target_pid_to_str,
which actually converts a ptid (not pid) to a string.

This is the string that appears (at least in part) in the output of
'info threads' in the 'Target Id' column, but also in the thread
exited message that GDB prints.

Having access to this string from Python is useful for allowing
extensions identify threads in a similar way to how GDB core would
identify the thread.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>