Go1.5 and go1.6 are not used anywhere in slapos tree and neither these versions are supported by upstream release policy https://golang.org/doc/devel/release.html#policy anymore. So let's remove them.

1.8.X is current stable series of Go.

https://golang.org/doc/go1.8
https://golang.org/doc/devel/release.html#go1.8

Besides many other significant improvements it should now also support ARM64.

/cc @jp
/reviewed-by TrustMe  (tested on helloworld)

Otherwise it started to fail this way:

[2017-02-22 11:49:22,982] INFO     --- FAIL: TestLoadFixed (0.00s)
[2017-02-22 11:49:22,982] INFO          time_test.go:929: Now().In(loc).Zone() = "-01", -3600, want "GMT+1", -3600
[2017-02-22 11:49:22,983] INFO     FAIL
[2017-02-22 11:49:22,983] INFO     FAIL time    2.380s

See https://github.com/golang/go/issues/16352#issuecomment-255835099 and around
for details.

From upcoming https://golang.org/doc/devel/release.html#go1.6.minor

    go1.6.3 (released 2016/07/17) includes security fixes to the net/http/cgi
    package and net/http package when used in a CGI environment. This release also
    adds support for macOS Sierra.  See the Go 1.6.3 milestone[1] on our issue
    tracker for details.

    [1] https://github.com/golang/go/issues?q=milestone%3AGo1.6.3

/reviewed-by TrustMe    (tested with helloworld)

1.6.X is current stable series of Go.

https://golang.org/doc/go1.6
https://golang.org/doc/devel/release.html#go1.6

So switch helloweb and default golang to it.

/reviewed-by TrustMe  (tested on helloworld)

From https://golang.org/doc/devel/release.html#go1.5.minor:

    go1.5.4 (released 2016/04/12) includes two security fixes. It contains
    the same fixes as Go 1.6.1 and was released at the same time. See the Go
    1.6.1 milestone[1] on our issue tracker for details.

    [1] https://github.com/golang/go/issues?q=milestone%3AGo1.6.1

/reviewed-by TrustMe    (tested with helloworld)

This is a security-bugfix release:

From https://golang.org/doc/devel/release.html#go1.5.minor:

    go1.5.3 (released 2016/01/13) includes a security fix to the math/big
    package affecting the crypto/tls package. See the Go 1.5.3 milestone on
    our issue tracker[1] and the release announcement[2] for details.

    [1] https://github.com/golang/go/issues?q=milestone%3AGo1.5.3
    [2] https://golang.org/s/go153announce

I've rebuild helloworld software release and briefly tested helloweb.go
with go 1.5.3 ok.

/reviewed-by: TrustMe
/cc @kazuhiko, !39

From https://golang.org/doc/devel/release.html#go1.5.minor:

    go1.5.2 (released 2015/12/02) includes bug fixes to the compiler,
    linker, and the mime/multipart, net, and runtime packages

/reviewed-by: TrustMe

Add support for Go language in the form of providing component for Go
toolchain and stdlib.

Current Go stable series is 1.5.* but to bootstrap it one need to have
Go 1.4.* available, which itself needs C compiler to bootstrap.

We need Go support for upcoming GitLab SR, where one server is written
in this language:

    https://gitlab.com/gitlab-org/gitlab-workhorse

/cc @gabriel, @Camata