fix pthread_create return value with PTHREAD_EXPLICIT_SCHED

fix pthread_create return value with PTHREAD_EXPLICIT_SCHED

due to moved code, commit b8742f32602add243ee2ce74d804015463726899
inadvertently used the return value of __clone, rather than the return
value of SYS_sched_setscheduler in the new thread, to check whether it
needed to report failure. since a successful __clone returns the tid
of the new thread, which is never zero, this caused pthread_create
always to return with an invalid error number in the code path for
PTHREAD_EXPLICIT_SCHED.

this regression was not present in any releases.