benchtests: Add random() benchmark

Add a simple benchmark to measure the overhead of internal libc locks in
the random() implementation on both single- and multi-threaded cases.
This relies on the implementation of random using internal locks to
access shared global data, and that the runtime uses multi-threaded
locking once a thread has been created (even after it finishes).

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>