This version doesn't have short reads. We'll eventually rename the
syscall back to zx_cprng_draw once all the clients have migrated to the
new semantics.

Change-Id: I7a7f6751e4d85dcc9b0a03a533dd93f3cbee277f
Reviewed-on: https://boringssl-review.googlesource.com/29084
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>