internal/syscall/unix: use our own version of this package

The upstream one assumes it's running on a Unix system (which makes
sense), but this package is also used on baremetal. So replace it on
systems that need a replaced syscall package.

src/internal/syscall/unix/constants.go

@@ -0,0 +1,7 @@
+package unix
+
+const (
+	R_OK = 0x4
+	W_OK = 0x2
+	X_OK = 0x1
+)