Replace random(), pg_erand48(), etc with a better PRNG API and algorithm.

Standardize on xoroshiro128** as our basic PRNG algorithm, eliminating
a bunch of platform dependencies as well as fundamentally-obsolete PRNG
code.  In addition, this API replacement will ease replacing the
algorithm again in future, should that become necessary.

xoroshiro128** is a few percent slower than the drand48 family,
but it can produce full-width 64-bit random values not only 48-bit,
and it should be much more trustworthy.  It's likely to be noticeably
faster than the platform's random(), depending on which platform you
are thinking about; and we can have non-global state vectors easily,
unlike with random().  It is not cryptographically strong, but neither
are the functions it replaces.

Fabien Coelho, reviewed by Dean Rasheed, Aleksander Alekseev, and myself

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2105241211230.165418@pseudo

src/backend/executor/nodeSamplescan.c

@@ -17,6 +17,7 @@
 #include "access/relscan.h"
 #include "access/tableam.h"
 #include "access/tsmapi.h"
+#include "common/pg_prng.h"
 #include "executor/executor.h"
 #include "executor/nodeSamplescan.h"
 #include "miscadmin.h"
@@ -154,7 +155,7 @@ ExecInitSampleScan(SampleScan *node, EState *estate, int eflags)
 	 * do this just once, since the seed shouldn't change over rescans.
 	 */
 	if (tsc->repeatable == NULL)
-		scanstate->seed = random();
+		scanstate->seed = pg_prng_uint32(&pg_global_prng_state);
 
 	/*
 	 * Finally, initialize the TABLESAMPLE method handler.