xtensa: Fix thread entry point

The stack initilaization was calling the user-provided entry function
directly, which works fine until that function returns, at which point
it will try to unspill A0-A3 from the 16 bytes above the allocated
stack and then "return" to a NULL pointer.

The kernel provides a _thread_entry() function that does cleanup
properly, so use that.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>