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/include/pg_config_manual.h

@@ -234,17 +234,6 @@
  */
 #define DEFAULT_EVENT_SOURCE  "PostgreSQL"
 
-/*
- * The random() function is expected to yield values between 0 and
- * MAX_RANDOM_VALUE.  Currently, all known implementations yield
- * 0..2^31-1, so we just hardwire this constant.  We could do a
- * configure test if it proves to be necessary.  CAUTION: Think not to
- * replace this with RAND_MAX.  RAND_MAX defines the maximum value of
- * the older rand() function, which is often different from --- and
- * considerably inferior to --- random().
- */
-#define MAX_RANDOM_VALUE  PG_INT32_MAX
-
 /*
  * On PPC machines, decide whether to use the mutex hint bit in LWARX
  * instructions.  Setting the hint bit will slightly improve spinlock