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/lib/bloomfilter.c

@@ -81,8 +81,7 @@ static inline uint32 mod_m(uint32 a, uint64 m);
  * distinct seed value on every call makes it unlikely that the same false
  * positives will reoccur when the same set is fingerprinted a second time.
  * Callers that don't care about this pass a constant as their seed, typically
- * 0.  Callers can use a pseudo-random seed in the range of 0 - INT_MAX by
- * calling random().
+ * 0.  Callers can also use a pseudo-random seed, eg from pg_prng_uint64().
  */
 bloom_filter *
 bloom_create(int64 total_elems, int bloom_work_mem, uint64 seed)