Use Syscall.Tcgetattr and Syscall.Tcsetattr of GCCGO when available

This PR fixes Issue https://github.com/docker/docker/issues/8653 (This issue seems closed by mistake. My previous PR https://github.com/docker/libcontainer/pull/231 does not fix this issue.)

The direct calls of ioctl for TCGETS and TCSETS do not work correctly on Power architectures as describe in Issue https://github.com/docker/docker/issues/8653.

We discuss this issue with the Go community, and the suggestion is using Syscall.Tcgetattr and Syscall.Tcsetattr available for GCCGO.
https://code.google.com/p/go/issues/detail?id=9010&thanks=9010&ts=1414521788

We use GCCGO to build Docker on Power, and keep using GC on x86_64, so I use build constraints to enable my code only when GCCGO is used on Linux.

Signed-off-by: Yohei Ueda <yohei@jp.ibm.com>
@tiborvass 
I don't have a good idea for the suffix, so I chose `_posix` just because `Tcgetattr` and `Tcsetattr` are defined in [`syscall/libcall_posix.go`](https://gcc.gnu.org/viewcvs/gcc/trunk/libgo/go/syscall/libcall_posix.go?view=markup) in GCCGO.

I can change the suffix for something more appropriate.