runtime: add runtime.rand function

This function is needed for Go 1.22, and is used from various packages
like math/rand.

When there is no random number generator available, it falls back to a
static sequence of numbers. I think this is fine, because as far as I
can see it is only used for non-cryptographic needs.

src/runtime/rand_norng.go

@@ -0,0 +1,7 @@
+//go:build baremetal && !(nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3)
+
+package runtime
+
+func hardwareRand() (n uint64, ok bool) {
+	return 0, false // no RNG available
+}