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/backend/optimizer/geqo/geqo_random.c

@@ -36,5 +36,5 @@ geqo_rand(PlannerInfo *root)
 {
 	GeqoPrivateData *private = (GeqoPrivateData *) root->join_search_private;
 
-	return erand48(private->random_state);
+	return pg_erand48(private->random_state);
 }

src/backend/optimizer/geqo/geqo_selection.c

@@ -64,9 +64,9 @@ geqo_selection(PlannerInfo *root, Chromosome *momma, Chromosome *daddy,
 	 * Ensure we have selected different genes, except if pool size is only
 	 * one, when we can't.
 	 *
-	 * This code has been observed to hang up in an infinite loop when the
-	 * platform's implementation of erand48() is broken.  We consider that a
-	 * feature: it lets you know you'd better fix the random-number generator.
+	 * This code was observed to hang up in an infinite loop when the
+	 * platform's implementation of erand48() was broken.  We now always
+	 * use our own version.
 	 */
 	if (pool->size > 1)
 	{