Fix printm on RV32 (#119)

Use a proxy syscall instead of a blocking character write.

Resolves #84

machine/htif.c

@@ -4,6 +4,7 @@
 #include "atomic.h"
 #include "mtrap.h"
 #include "fdt.h"
+#include "syscall.h"
 #include <string.h>
 
 extern uint64_t __htif_base;
@@ -48,6 +49,11 @@ static void __set_tohost(uintptr_t dev, uintptr_t cmd, uintptr_t data)
 
 int htif_console_getchar()
 {
+#if __riscv_xlen == 32
+  // HTIF devices are not supported on RV32
+  return -1;
+#endif
+
   spinlock_lock(&htif_lock);
     __check_fromhost();
     int ch = htif_console_buf;
@@ -85,9 +91,19 @@ void htif_syscall(uintptr_t arg)
 
 void htif_console_putchar(uint8_t ch)
 {
+#if __riscv_xlen == 32
+  // HTIF devices are not supported on RV32, so proxy a write system call
+  volatile uint64_t magic_mem[8];
+  magic_mem[0] = SYS_write;
+  magic_mem[1] = 1;
+  magic_mem[2] = (uintptr_t)&ch;
+  magic_mem[3] = 1;
+  do_tohost_fromhost(0, 0, (uintptr_t)magic_mem);
+#else
   spinlock_lock(&htif_lock);
     __set_tohost(1, 1, ch);
   spinlock_unlock(&htif_lock);
+#endif
 }
 
 void htif_poweroff()