Xtensa port: _thread_entry shall use call4 to start the thread.

The function _new_thread sets the CALLINC for _thread_entry when calling the
new thread entry point. This should be CALLINC(1) (call4) instead of previous
one CALLINC(2) (call8).

This change allows resolving a crash when starting the first thread.

Change-Id: Ie97c95c87d1219342fed09f670fcae8a6230cefe
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>