Update Go to 1.7.

The latest Go release, version 1.7, arrives six months after 1.6. Most of its
changes are in the implementation of the toolchain, runtime, and libraries.
There is one minor change to the language specification. As always, the release
maintains the Go 1 promise of compatibility. We expect almost all Go programs
to continue to compile and run as before.

There is one tiny language change in this release. The section on terminating
statements clarifies that to determine whether a statement list ends in a
terminating statement, the ���final non-empty statement��� is considered the end,
matching the existing behavior of the gc and gccgo compiler toolchains. In
earlier releases the definition referred only to the ���final statement,��� leaving
the effect of trailing empty statements at the least unclear. The go/types
package has been updated to match the gc and gccgo compiler toolchains in this
respect. This change has no effect on the correctness of existing programs.

Go 1.7 adds support for macOS 10.12 Sierra. This support was backported to Go
1.6.3. Binaries built with versions of Go before 1.6.3 will not work correctly
on Sierra.

(bsiegert)