Merge #93

93: Rerun the build script if the bin/{}.a archive has changed r=Disasm a=SimonSapin

This PR also includes some drive-by cleanup commits, but the last one is what’s significant. This bit me when trying to make changes to `asm.S` and rebuilding with `./assemble.sh`.

(By the way, I get ``Assembler messages: Fatal error: invalid -march= option: `rv32i'`` when running that script on Arch Linux despite installing a GNU toolchain for RISC-V. Instead I’m now using an Ubuntu 20.04 container to run it, since that’s what CI does for `./check-blobs.sh`.)

Co-authored-by: Simon Sapin <simon.sapin@exyr.org>