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/runtime/os_darwin.c

@@ -0,0 +1,7 @@
+// Wrapper function because 'open' is a variadic function and variadic functions
+// use a different (incompatible) calling convention on darwin/arm64.
+
+#include <fcntl.h>
+int syscall_libc_open(const char *pathname, int flags, mode_t mode) {
+    return open(pathname, flags, mode);
+}

src/runtime/os_darwin.go

@@ -5,6 +5,8 @@ package runtime
 
 import "unsafe"
 
+import "C" // dummy import so that os_darwin.c works
+
 const GOOS = "darwin"
 
 const (