x/build: update NetBSD builders to NetBSD 8-beta

[bradfitz]

In https://go-review.googlesource.com/c/47036/, Christos Zoulas mentions that there are known bugs in the NetBSD 7.1 kqueue code and we should switch our builders to NetBSD 8-beta.

Also mentioned in #20836 (comment)

This is a tracking bug to update our NetBSD builders past 7.1

I've updated https://golang.org/wiki/MinimumRequirements too.

/cc @krytarowski @bsiegert @zoulasc @adams-sarah @andybons

[zoulasc]

This should also be fixed in the 7.1 branch now, builds available from:
https://releng.netbsd.org/builds/netbsd-7-1/201707090640Z/

[bsiegert]

I would be more comfortable with a netbsd-8 system than with 7.1. http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=52034 is still pending, and it means that significant disk activity is likely to crash the VM, or make it hang.

[gopherbot]

CL https://golang.org/cl/48091 mentions this issue.

[gopherbot]

CL https://golang.org/cl/48552 mentions this issue.

[bradfitz]

@bsiegert, this doesn't boot on GCE as far as I can tell:

$ gomote create netbsd-amd64-8branch
Error running create: failed to create buildlet: 500 Internal Server Error: buildlet didn't come up at http://10.240.0.21 in 5m0s

Please let me know once you've actually run it on GCE.

Until that time, Go will continue to not run NetBSD builders.

[bradfitz]

I didn't mean to close this. Reopened.