riscv: fall back to syscall __riscv_flush_icache

Matches glibc behavior and fixes a case where we could fall off the
function without returning a value.

src/linux/cache.c

@@ -45,6 +45,7 @@ int __riscv_flush_icache(void *start, void *end, unsigned long int flags)
 		if (!r) return r;
 		if (r != -ENOSYS) return __syscall_ret(r);
 	}
+	return syscall(SYS_riscv_flush_icache, start, end, flags);
 }
 weak_alias(__riscv_flush_icache, riscv_flush_icache);
 #endif