Created attachment 213063 [details] patch
I don't have an arm to test this on, but it does seem right by the look of it.
A commit references this bug: Author: mikael Date: Thu Apr 16 15:16:30 UTC 2020 New revision: 531855 URL: https://svnweb.freebsd.org/changeset/ports/531855 Log: net-mgmt/alertmanager: update 0.20.0 - update 0.20.0 - fix build on aarch64 PR: 245350 Approved by: michal@hanu.la (maintainer) Changes: head/net-mgmt/alertmanager/Makefile head/net-mgmt/alertmanager/distinfo head/net-mgmt/alertmanager/files/patch-vendor_golang.org_x_net_ipv4_zsys__freebsd__arm64.go head/net-mgmt/alertmanager/files/patch-vendor_golang.org_x_net_ipv6_zsys__freebsd__arm64.go head/net-mgmt/alertmanager/files/patch-vendor_golang.org_x_sys_unix_ztypes__freebsd__arm64.go