Since the 12.x branch, the getrandom syscall had been introduced
with similar interface as Linux's and consistent syscall id
across architectures.

Change-Id: I63d6b45dbe9e29f07f1b5b6c2ec8be4fa624b9ee
GitHub-Last-Rev: 6fb76e6522ef5ccb96d02445ffa39796dae89016
GitHub-Pull-Request: golang/go#25976
Reviewed-on: https://go-review.googlesource.com/120055
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>