Cross compiling to darwin and openbsd not working

[matm]

Not working after using the new prompt added in #43.

• For darwin, the golang.org/x/sys/unix package fails to compile

../go/pkg/mod/golang.org/x/sys@v0.0.0-20200918174421-af09f7315aff/unix/syscall_darwin.1_13.go:29:3: //go:linkname must refer to declared function or variable
../go/pkg/mod/golang.org/x/sys@v0.0.0-20200918174421-af09f7315aff/unix/zsyscall_darwin_amd64.1_13.go:27:3: //go:linkname must refer to declared function or variable
...

• For openbsd, the github.com/pkg/term/termios package fails to compile

../go/pkg/mod/github.com/pkg/term@v1.2.0-beta.2/termios/pty.go:23:14: undefined: open_pty_master
../go/pkg/mod/github.com/pkg/term@v1.2.0-beta.2/termios/pty.go:28:16: undefined: Ptsname
../go/pkg/mod/github.com/pkg/term@v1.2.0-beta.2/termios/pty.go:33:8: undefined: grantpt
../go/pkg/mod/github.com/pkg/term@v1.2.0-beta.2/termios/pty.go:38:8: undefined: unlockpt

[matm]

Fixed by #50