x/build: run ssacheck mode for all architectures, not just amd64

[josharian]

We have an amd64 ssacheck builder. That builder should compile std+cmd for all architectures, not just native. That would have prevented or at least caught #24014.

[bradfitz]

We already have a bunch of misc-compile builders. Can we just make them ssacheck too?

[josharian]

SGTM. It’ll make them a bit slower, but that seems ok.