x/build: get more coverage with the ssacheck builder

[josharian]

The ssacheck builder currently builds only amd64, and with a fixed random seed.

We should improve on both dimensions.

• CL 216418 lets us provide a random seed. We could generate the random seed and print it to stdout before using it, so that any failure will be reproducible from the log.
• We should either run on all architectures, or pick one at random each time from go tool dist list (and log it, for reproducibility).

I suspect that doing both of these things is pretty easy, and it'd get us extra test coverage, so it'd be nice to do soon. This would have caught both #36723 and #36755.

cc @cherrymui @randall77 @dmitshur @bradfitz

[josharian]

Another this would have caught: #38880