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/crypto/rand/rand_baremetal.go

@@ -1,4 +1,7 @@
-//go:build nrf || stm32 || (sam && atsamd51) || (sam && atsame5x) || esp32c3
+//go:build nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3
+
+// If you update the above build constraint, you'll probably also need to update
+// src/runtime/rand_hwrng.go.
 
 package rand