While I don't believe EINTR can occur with a non-blocking getrandom call
when talking to the kernel directly, that may not be true when certain
sandboxing systems are being used.
Additionally, with this change we will no longer silently ignore errors
other than ENOSYS.
Update-Note: update internal bug 115344138.
Change-Id: I952c132cf325dcc17dc38e68f054abc41de1f8b0
Reviewed-on: https://boringssl-review.googlesource.com/32006
Reviewed-by: David Benjamin <davidben@google.com>