.goxc.json: list os/arch explicitly to avoid darwin/386

[peat-psuwit]

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Go 1.15 drops support for darwin/386 GOOS/GOARCH pair [1]. So, we have
to skip this pair, and thus cannot use simple os multiply arch anymore.
Switch to goxc's BuildConstraints config, which uses the same syntax as
Go's build constraint header [2].

[1] golang/go#37610
[2] https://golang.org/cmd/go/#hdr-Build_constraints

Fixes #943

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

Merging #947 (7db7e3c) into master (cbf0416) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #947   +/-   ##
=======================================
  Coverage   61.28%   61.28%           
=======================================
  Files          54       54           
  Lines        5928     5928           
=======================================
  Hits         3633     3633           
  Misses       1770     1770           
  Partials      525      525           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cbf0416...7db7e3c. Read the comment docs.

[Smarre]

Looks good to me.

[mariogrip]

ping @lbolla