Backport a patch just committed to gc tip.  There is a bug report on
the golang-dev mailing list
(https://groups.google.com/d/msg/golang-dev/sDg-t1_DPw0/-AJmLxgPBQAJ)
in which waitid running on MIPS returns EFAULT, and this patch may fix
the problem.  Bootstrapped and ran Go tests on x86_64-pc-linux-gnu.
Committed to mainline and GCC 7 branch.

Ian