stdlib: Add arc4random tests

stdlib: Add arc4random tests

The basic tst-arc4random-chacha20.c checks if the output of ChaCha20
implementation matches the reference test vectors from RFC8439.

The tst-arc4random-fork.c check if subprocesses generate distinct
streams of randomness (if fork handling is done correctly).

The tst-arc4random-stats.c is a statistical test to the randomness of
arc4random, arc4random_buf, and arc4random_uniform.

The tst-arc4random-thread.c check if threads generate distinct streams
of randomness (if function are thread-safe).

Checked on x86_64-linux-gnu, aarch64-linux, and powerpc64le-linux-gnu.

Co-authored-by: Florian Weimer <fweimer@redhat.com>
Checked on x86_64-linux-gnu and aarch64-linux-gnu.