Update go19 to 1.9.7, latest on that branch.

go1.9.5 (released 2018/03/28) includes fixes to the compiler, go command, and
net/http/pprof package. See the Go 1.9.5 milestone on our issue tracker for
details.

go1.9.6 (released 2018/05/01) includes fixes to the compiler and go command.
See the Go 1.9.6 milestone on our issue tracker for details.

go1.9.7 (released 2018/06/05) includes fixes to the go command, and the
crypto/x509, and strings packages. In particular, it adds minimal support to
the go command for the vgo transition. See the Go 1.9.7 milestone on our issue
tracker for details.

(bsiegert)