Restore ARMv7 release builds

[slepp]

Summary

• use the libevent callback length type portably on 32-bit ARM
• build every release architecture in pull request CI so ARM regressions are caught before merge

Context

The first post-merge Docker Hub publish exposed an ARMv7-only evbuffer_add_reference callback type mismatch. amd64 and arm64 use an unsigned long-sized size_t, while ARMv7 uses unsigned int.

Validation

• make test
• CI static build matrix for amd64, arm64, and ARMv7