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/nbtree/nbtinsert.c

@@ -19,6 +19,7 @@
 #include "access/nbtxlog.h"
 #include "access/transam.h"
 #include "access/xloginsert.h"
+#include "common/pg_prng.h"
 #include "lib/qunique.h"
 #include "miscadmin.h"
 #include "storage/lmgr.h"
@@ -965,7 +966,7 @@ _bt_findinsertloc(Relation rel,
 
 			if (P_RIGHTMOST(opaque) ||
 				_bt_compare(rel, itup_key, page, P_HIKEY) != 0 ||
-				random() <= (MAX_RANDOM_VALUE / 100))
+				pg_prng_uint32(&pg_global_prng_state) <= (PG_UINT32_MAX / 100))
 				break;
 
 			_bt_stepright(rel, insertstate, stack);