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.

loader/goroot.go

@@ -269,6 +269,8 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 
 	if needsSyscallPackage {
 		paths["syscall/"] = true // include syscall/js
+		paths["internal/syscall/"] = true
+		paths["internal/syscall/unix/"] = false
 	}
 	return paths
 }