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

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

.goxc.json

@@ -34,8 +34,7 @@
         }
     },
 	"ResourcesInclude": "README.rst,LICENSE,AUTHORS,man/aptly.1",
-	"Arch": "386 amd64",
-	"Os": "linux darwin freebsd",
+	"BuildConstraints": "linux,386 linux,amd64 darwin,amd64 freebsd,386 freebsd,amd64",
 	"MainDirsExclude": "_man,vendor",
 	"BuildSettings": {
 		"LdFlagsXVars": {