This change updates the GOARCH/GOOS discussion at the top of the
"Installing Go from source" document to better reflect the current
status. In particular:

- The GOARCH list now focuses on simply listing the supported
architectures, with no notes about their supposed "maturity", since
the same GOARCH can be mature on a GOOS and not so mature on another.

- Outdated notes about some archs being new and "not well-exercised"
have been removed in favour of a following list of which ports are
first class.

- The list of supported OS has been updated (added: AIX, Illumos),
and sorted in alphabetical order.

- A note about the runtime support being the same for all ARCHS,
"including garbage collection and efficient array slicing and" etc etc
has been removed, since it doesn't seem particularly relevant in a
"install from source" instruction page, and it's likely a leftover
from the time this doc page was the landing place for new people and
it felt the need to "sell" Go.

Updates #27689
Fixes #35009

Change-Id: Ic4eca91dca3135adc7bed4fe00b4f157768f0e81
Reviewed-on: https://go-review.googlesource.com/c/go/+/202197Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>