Summary:
[Folly] Use `getrandom` instead of `/dev/urandom` when available.

The C library function `getrandom` is available on linux >= 3.17 and glibc >= 2.25.

Reviewed By: ot, simpkins, luciang

Differential Revision: D19211519

fbshipit-source-id: 07e7e856fc81d1f04efca4a6cfa0b76ada08f443