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.go

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