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/runtime_tinygowasm.go

@@ -85,3 +85,14 @@ func procPin() {
 //go:linkname procUnpin sync/atomic.runtime_procUnpin
 func procUnpin() {
 }
+
+func hardwareRand() (n uint64, ok bool) {
+	n |= uint64(libc_arc4random())
+	n |= uint64(libc_arc4random()) << 32
+	return n, true
+}
+
+// uint32_t arc4random(void);
+//
+//export arc4random
+func libc_arc4random() uint32