build: provide clear instructions + workflow for folk who want to build crdb on non-supported platforms

We have customers running crdb on non-supported platforms, and these customers will translate to future revenue.

These include currently (But will not be limited to):
- linux on IBM s/390
- illumos on oxide

Generally, we want to continue tier-3 support for any architecture supported by the go and c++ compiler. This includes the two above and will include other platforms we've not considered yet.

Reminder of support tiers for open source projects:
- tier-1: undergoing QA, validated using tests in CI, supported in paid contract
- tier-2: build success verified in CI, but not via automated testing, no paid support
- tier-3: build verified manually, usually by community

Currently crdb has a tier-1, tier-2 and **also a tier-3 story via the makefile and manual changes to the make rules**

if we remove the make story, we need to replace it by a dev-based story that continue support for tier-3.

xref #93162 for an example

Jira issue: CRDB-22198