libgolang: random_shuffle -> shuffle

There is no std::random_shuffle on `MSVC /std:c++20` because it was removed in C++17:

    golang/runtime/libgolang.cpp(946): error C2039: 'random_shuffle': is not a member of 'std'
    z:\home\kirr\src\tools\go\pygo-win\BuildTools\vc\tools\msvc\14.35.32215\include\random(34): note: see declaration of 'std'
    golang/runtime/libgolang.cpp(946): error C3861: 'random_shuffle': identifier not found

    https://en.cppreference.com/w/cpp/algorithm/random_shuffle

golang/runtime/libgolang.cpp

@@ -900,6 +900,13 @@ template<> int _chanselect2</*onstack=*/true> (const _selcase *, int, const vect
 template<> int _chanselect2</*onstack=*/false>(const _selcase *, int, const vector<int>&);
 static int __chanselect2(const _selcase *, int, const vector<int>&, _WaitGroup*);
 
+// PRNG for select.
+// TODO consider switching to xoroshiro or wyrand if speed is an issue
+// https://thompsonsed.co.uk/random-number-generators-for-c-performance-tested
+// https://nullprogram.com/blog/2017/09/21/
+static std::random_device        _devrand;
+static thread_local std::mt19937 _t_rng(_devrand());
+
 // _chanselect executes one ready send or receive channel case.
 //
 // if no case is ready and default case was provided, select chooses default.
@@ -926,7 +933,7 @@ int _chanselect(const _selcase *casev, int casec) {
     vector<int> nv(casec); // n -> n(case)      TODO -> caller stack-allocate nv
     for (int i=0; i <casec; i++)
         nv[i] = i;
-    std::random_shuffle(nv.begin(), nv.end());
+    std::shuffle(nv.begin(), nv.end(), _t_rng);
 
     // first pass: poll all cases and bail out in the end if default was provided
     int  ndefault = -1;