Make pgbench use erand48() rather than random().

glibc renders random() thread-safe by wrapping a futex lock around it;
testing reveals that this limits the performance of pgbench on machines
with many CPU cores.  Rather than switching to random_r(), which is
only available on GNU systems and crashes unless you use undocumented
alchemy to initialize the random state properly, switch to our built-in
implementation of erand48(), which is both thread-safe and concurrent.

Since the list of reasons not to use the operating system's erand48()
is getting rather long, rename ours to pg_erand48() (and similarly
for our implementations of lrand48() and srand48()) and just always
use those.  We were already doing this on Cygwin anyway, and the
glibc implementation is not quite thread-safe, so pgbench wouldn't
be able to use that either.

Per discussion with Tom Lane.

src/include/optimizer/geqo.h

@@ -73,7 +73,7 @@ extern double Geqo_seed;		/* 0 .. 1 */
 typedef struct
 {
 	List	   *initial_rels;	/* the base relations we are joining */
-	unsigned short random_state[3];		/* state for erand48() */
+	unsigned short random_state[3];		/* state for pg_erand48() */
 } GeqoPrivateData;