arm64: Implement __clear_cache.

__clear_cache is defined in lib-arm64.c with a single call to
__arm64_clear_cache, which is the real built-in function and is
turned into inline assembler by gen_clear_cache in arm64-gen.c

tccgen.c

@@ -3979,6 +3979,18 @@ ST_FUNC void unary(void)
         vtop->type = type;
         break;
     }
+    case TOK___arm64_clear_cache: {
+        next();
+        skip('(');
+        expr_eq();
+        skip(',');
+        expr_eq();
+        skip(')');
+        gen_clear_cache();
+        vpushi(0);
+        vtop->type.t = VT_VOID;
+        break;
+    }
 #endif
 
     case TOK_INC: