doc: outdated assessment of ARCHs maturity in "Installing Go from source" list

[ALTree]

The Installing Go from source document contains, in the introduction section, a list of supported architectures. It says:

The Go compilers support nine instruction sets. There are important differences in the quality of the compilers for the different architectures.

The assessment of the various ports' "maturity grade" is outdated and somewhat misleading. A few issues:

• s390x is listed as "new" and "not as well exercised as other ports."; but in reality is probably one of the ports that receive the most attention.
• the arm64 port is mature, but it is listed as "not as well exercised as other ports"; it also says "New in 1.5", it seems unnecessary to note the newness of something that is 4 years old.
• for 386 it says "Comparable to the amd64 port" (which is the most mature one), but in reality 386 is probably much less exercised than ports like arm / arm64 / s390x.
• In general, the point of the list is not to present a chronological history of new ports, but to help the reader assess the newness and maturity of the archs. For this reason, mentioning that mips64 is "New in 1.6" is not useful (at this point, anything introduced in 1.6 is quite old).

I suggest we update the list to reflect the current status of the various ports. I'll send a CL.

[gopherbot]

Change https://golang.org/cl/202197 mentions this issue: doc: update ports list description to reflect current status