darwin: fix syscall.Open on darwin/arm64

Unfortunately the calling convention for variadic functions is different from
the calling convention of regular functions on darwin/arm64, and open happens
to be such a variadic function. The syscall package treated it like a regular
function, which resulted in buggy behavior.

This fix introduces a wrapper function. This is the cleanest change I could
come up with.

src/syscall/syscall_libc_nintendoswitch.go

@@ -71,3 +71,8 @@ func Pipe2(p []int, flags int) (err error) {
 func Getpagesize() int {
 	return 4096 // TODO
 }
+
+// int open(const char *pathname, int flags, mode_t mode);
+//
+//export open
+func libc_open(pathname *byte, flags int32, mode uint32) int32