This breaks our dependency on the x/tools repository, which has a
tendency to change in unexpected ways. It also means we can use the
version of go/types that ships with Go 1.5.
Along the way, it appears that cgo processing has changed slightly.
The old check for cgo files apparently wasn't working, so I removed
it.
Change-Id: I14378e9df9cd65c5ab61b47728ba0d56f31cdf76
Reviewed-on: https://go-review.googlesource.com/12680
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
