Wrap built-in function with wrapper function

wrap __builtin_ffs with static inline ffs function.

Author: ranvd

common.h

@@ -17,6 +17,11 @@ static inline int ilog2(int x)
     return 31 - __builtin_clz(x | 1);
 }
 
+static inline int ffs(int x)
+{
+    return __builtin_ffs(x);
+}
+
 /* Range check
  * For any variable range checking:
  *     if (x >= minx && x <= maxx) ...

riscv.c

@@ -806,7 +806,7 @@ void vm_step(hart_t *vm)
     vm->current_pc = vm->pc;
     if ((vm->sstatus_sie || !vm->s_mode) && (vm->sip & vm->sie)) {
         uint32_t applicable = (vm->sip & vm->sie);
-        uint8_t idx = __builtin_ffs(applicable) - 1;
+        uint8_t idx = ffs(applicable) - 1;
         if (idx == 1) {
             emu_state_t *data = PRIV(vm);
             data->clint.msip[vm->mhartid] = 0;