moves to-/fromhost declaration into platform

env/iss/platform.h

@@ -22,6 +22,9 @@
 
 #include <stdint.h>
 
+extern volatile uintptr_t tohost;
+extern volatile uintptr_t fromhost;
+
 void init_pll(void);
 unsigned long get_cpu_freq(void);
 unsigned long get_timer_freq(void);