std::rand::OsRng: Use `getrandom` syscall on Linux

`getrandom(2)` system call [1] has been added on Linux 3.17.
This patch makes `OsRng` use `getrandom` if available, and use
traditional `/dev/urandom` fallback if not.

[1]:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c6e9d6f38894798696f23c8084ca7edbf16ee895