remove per-OS arc4random_buf overrides

If an OS provides an arc4random_buf implementation in its C library,
prefer it over an in-library version. This allows OS-specific
implementations to become more robust over time. It also prevents
possible link-time confusion as to which arc4random_buf implementation
is in use by an application when linked with LibreSSL.

The built-in unit tests will identify some common issues, such as fork
safety and PID wrap handling. Other elements, such as seeding
mechanisms, should be audited by the vendor or user for correctness.

ok deraadt@ beck@