gdb/python: add gdb.Value.is_unavailable attribute

Add a new gdb.Value.is_unavailable attribute.  This is similar to the
existing Value.is_optimized_out attribute, but returns True if any
part of the value is <unavailable>.

The existing Value.is_optimized_out attribute returns true if any part
of the value is optimized out, so I thought that Value.is_unavailable
should work the same way.

There's also a test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33345

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>