Diff reduce with upstream and prep for next set of content pushes

fpu: ensure calls to pthread_create succeed and test sched_yield to make
sure it returns 0

sched_yield tests for values returning 0 of type int and sched_yield is
of type long, so the test is a mismatch

MFC after: 1 week
Reported by: Coverity
CID: 1254953, 1254954, 1254965, 1254966