Modernize the PRNG used for rand() by using a Configure option

[scottchiefbaker]

Per past discussion on p5p we know the drand48 we use as a PRNG has limitations. This PR attempts to cleanly upgrade the PRNG used for rand() calls, while leaving the existing drand48 code in place for internal Perl stuff (hash seed, etc). I found randfunc and seedfunc options in Configure that allow us to point at a new PRNG without affecting the existing code.

I created a new prng.h file which is largely self-contained except for a couple of additions to embed.fnc and Configure. I have included two new PRNG implementations: PCG64 (my preference) and xoroshiro128** as options.

Completed items

• Modern PRNG implementation (PCG64)
• Detailed instructions for future devs on how to extend and modify
• Updated unit tests
• Verify srand() functionality works as expected
• Verify the new rand() outputs the full 53 bit state capable from a double (drand48 could only do 48 bits)
  • ./perl -I lib -E 'for (1..5) { printf("%064b\n", rand() * 2**64-1); }'

TODO

• prng.h does not seem to be rebuilt consistently after changes. Do I need to add this new file to build system?
• Bikeshed on what the best PRNG is in 2026
• make regen puts the functions prototypes in a weird location "Used in locale.c and perl.c"
• Add an option to get a random integer? rand64()?

Alternate options

• We don't do anything. rand() is "good enough"
• Point users at CPAN. Random::Simple is a drop-in replacement for rand() and srand() already

[tonycoz]

This isn't thread-safe.

[Leont]

Yeah, this should probably live in the interpreter instead. And do something sensible on thread cloning so different threads don't get the same results.