crypto/rand: use new getrandom system call on Linux

[bradfitz]

Linux has a new getrandom system call:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c6e9d6f38894798696f23c8084ca7edbf16ee895

We could use it in crypto/rand.

[gopherbot]

Comment 1:

CL https://golang.org/cl/123260044 mentions this issue.

[bradfitz]

Comment 2:

This issue was closed by revision 67e1d40.

Status changed to Fixed.