cgo: support errno value as second return parameter

Making this work on all targets was interesting but there's now a test
in place to make sure this works on all targets that have the CGo test
enabled (which is almost all targets).

testdata/cgo/out.txt

@@ -75,6 +75,8 @@ len(C.GoStringN(nil, 0)): 0
 len(C.GoBytes(nil, 0)): 0
 len(C.GoBytes(C.CBytes(nil),0)): 0
 rountrip CBytes: hello
+EINVAL: true
+EAGAIN: true
 copied string: foobar
 CGo sqrt(3): +1.732051e+000
 C   sqrt(3): +1.732051e+000