libgo: update to Go 1.8.1 release

libgo/MERGE

libgo/VERSION

libgo/go/cmd/go/go_test.go

libgo/go/cmd/go/pkg.go

libgo/go/cmd/go/test.go

libgo/go/crypto/tls/common.go

libgo/go/crypto/tls/tls_test.go

libgo/go/encoding/xml/marshal_test.go

libgo/go/encoding/xml/read.go

libgo/go/encoding/xml/xml_test.go

libgo/go/image/png/reader.go

libgo/go/image/png/reader_test.go

libgo/go/internal/testenv/testenv.go

libgo/go/internal/testenv/testenv_cgo.go

libgo/go/net/http/http.go

libgo/go/net/net.go

libgo/go/os/exec/exec_test.go

libgo/go/reflect/all_test.go

libgo/go/runtime/crash_unix_test.go

libgo/go/runtime/export_test.go