runtime: manually initialize xorshift state

This ensures:

 1. The xorshift state is initialized during interp.
 2. The xorshift state gets initialized to a real random number on
    hardware that supports it at runtime.

This fixes a big binary size regression from the previous commit. It's
still not perfect: most programs increase binary size by a few bytes.
But it's not nearly as bad as before.

builder/sizes_test.go

@@ -43,8 +43,8 @@ func TestBinarySize(t *testing.T) {
 	tests := []sizeTest{
 		// microcontrollers
 		{"hifive1b", "examples/echo", 4560, 280, 0, 2268},
-		{"microbit", "examples/serial", 2908, 388, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7293, 1487, 116, 6912},
+		{"microbit", "examples/serial", 2916, 388, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 7315, 1489, 116, 6912},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the