k_system_control: Fix data race

`return distribution(gen)` is a data race between a read and a write in
two threads, reported by TSan. Remove static random number generators so
they aren't using the same generator.

src/core/hle/kernel/board/nintendo/nx/k_system_control.cpp

@@ -148,9 +148,9 @@ u64 GenerateUniformRange(u64 min, u64 max, F f) {
 } // Anonymous namespace
 
 u64 KSystemControl::GenerateRandomU64() {
-    static std::random_device device;
+    std::random_device device;
-    static std::mt19937 gen(device());
+    std::mt19937 gen(device());
-    static std::uniform_int_distribution<u64> distribution(1, std::numeric_limits<u64>::max());
+    std::uniform_int_distribution<u64> distribution(1, std::numeric_limits<u64>::max());
     return distribution(gen);
 }