This commit adds support for AIX operating system.

 - move term_solaris.go to term_nosyscall6.go. AIX like solaris doesn't
provide syscall.Syscall6 and must rely on x/sys/unix in order to perform
syscalls.

 - This patch won't work with versions prior to 1.13 because it needs
some constants added by http://go-review.googlesource.com/c/go/+/171339.