doc: install and install-source mentions superfluous OS and ARCHes for go1.5

[marcellanz]

Go1.5 Release Notes [2] notes some updates for available ports. some documents still mention then unavailable ports or their availability as a "first class port".

proposal:
[a] cleanup doc/install.html and remove "freebsd,386" "freebsd,arm" and "darwin,386" as an "installable" distribution
[b] update mention of "OS X 10.6" to "OS X 10.7" in [4,5] => 10.6 support as removed in [6]
[c] not that urgent perhaps, but redefine what is a "first class port", what is buildable/maintained by source but not a "first class port" and provide binaries for all "first class ports" via doc/install => golang.org/dl

doc/install.html mentions "The gc compiler supports the following operating systems and architectures."
this list is not complete in terms of supported OS and ARCHes by the compiler, but in the context of an installation document relevant what is downloadable.

notes:
The following is mentioned at different places in the documentation.
"required systems" (first class ports? [0]) are mentioned in doc/install.html [4]:
FreeBSD 8 or later amd64, 386, arm
Linux 2.6.23 or later with glibc amd64, 386, arm
Mac OS X 10.6 or later amd64, 386
Windows XP or later amd64, 386

the build dashboard [1] mentions "show only first-class ports" and shows a list of these ports:
darwin: x64 arm arm64
freebsd: 386 x64 arm
linux: 386 x64 arm arm64 ppc64 ppc64le
windows: 386 x64
android: arm

[0] says about "first class ports":

• "Broken builds block releases"
• "Official binaries are provided"
• "Installation is documented at http://golang.org/doc/install"

install.html should mention what is downloadable by the Go project.
If I install Go and read doc/install.html, whats listed there should be downloadable as a binary.
the download page [3] lists:
darwin,amd64
freebsd,amd64
linux,386
linux,amd64
windows,386
windows,amd64

as downloadable. "first class ports" shall provide binaries but trying to download "linux,ppc64" fails:
https://storage.googleapis.com/golang/go1.5beta3.linux-ppc64.tar.gz
also:
https://storage.googleapis.com/golang/go1.5beta3.darwin-arm64.tar.gz
does.

so the "first class ports" mentioned on [1] do not conform with the properties defined in [0].
I suspect the dash-board "first class port" short list, hides only OSes and not ARCHs (except for android).

[0] https://github.com/golang/go/wiki/PortingPolicy
[1] http://build.golang.org/#short
[2] http://tip.golang.org/doc/go1.5#ports
[3] https://golang.org/dl
[4] https://tip.golang.org/doc/install
[5] https://tip.golang.org/doc/install/source
[6] #9511

[gopherbot]

CL https://golang.org/cl/13071 mentions this issue.

[gopherbot]

CL https://golang.org/cl/13150 mentions this issue.