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_darwin.go

@@ -281,3 +281,8 @@ func libc_pipe(fds *int32) int32
 //
 //export getpagesize
 func libc_getpagesize() int32
+
+// int open(const char *pathname, int flags, mode_t mode);
+//
+//export syscall_libc_open
+func libc_open(pathname *byte, flags int32, mode uint32) int32