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/access/spgist/spgdoinsert.c

@@ -19,6 +19,7 @@
 #include "access/spgist_private.h"
 #include "access/spgxlog.h"
 #include "access/xloginsert.h"
+#include "common/pg_prng.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "utils/rel.h"
@@ -2210,7 +2211,9 @@ spgdoinsert(Relation index, SpGistState *state,
 				if (out.resultType == spgAddNode)
 					elog(ERROR, "cannot add a node to an allTheSame inner tuple");
 				else if (out.resultType == spgMatchNode)
-					out.result.matchNode.nodeN = random() % innerTuple->nNodes;
+					out.result.matchNode.nodeN =
+						pg_prng_uint64_range(&pg_global_prng_state,
+											 0, innerTuple->nNodes - 1);
 			}
 
 			switch (out.resultType)