PR go/86331
os: check return value as well as error from waitid
https://gcc.gnu.org/PR86331 indicates that if a signal handler runs it
is possible for syscall.Syscall6 to return a non-zero errno value even
if no error occurs. That is a problem in general, but this fix will
let us work around the general problem for the specific case of
calling waitid.
Reviewed-on: https://go-review.googlesource.com/121595
From-SVN: r262313
