x86: don't pretend that non-framepointer stack traces are reliable

Without frame pointers enabled, the x86 stack traces should not
pretend to be reliable; instead they should just be what they are:
unreliable.

The effect of this is that they have a '?' printed in the stacktrace,
to warn the reader that these entries are guesses rather than known
based on more reliable information.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>