loader.py: fix thread_context for stacks in high end

This fixes 'osv info threads' command.

when $rsp holds 0xffffc0003fdf6fc8, ulong() for gdb.Value representing
it returns 18446673706036981704. This is out of int range. This
causes that thread_context.__exit__() throws "Numeric constant too
large" exception from

  set $rbp = 18446673706036981704

The fix removes the additional conversion as it seems to be
unnecessary. The gdb.Value should format itself properly.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>