xtensa: always use correct stack pointer for stack traces

xtensa: always use correct stack pointer for stack traces

Commit '28a0ce7 xtensa: use correct stack pointer for stack traces'
changed the stack tracer from always reading the stack pointer
register to always using the saved value in the task descriptor.

The author was too dense to consider the fact that the saved stack
value is stale for a running process und thus unusable for 'current'.

What we do now is to use the stack pointer register (a1) for when the
task is unknown - we can't help it then - or when the task is
'current'.  For everything else use the saved stack pointer value
contained in the task descriptor.

Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>