Add CI job to build C sources with various compiler versions and fix array-bounds error with GCC 9

[eregon]

From #4051 (review)

[kddnewton]

What's the deal with the changes to parser.h here?

[eregon]

@kddnewton Could you check the commit message? It explains it.

[eregon]

The new job took 10min15s, which is faster than memcheck.
So maybe it's fast enough for now?

Not sure where the time is spent but I wondered about image size:

$ docker images
ghcr.io/ruby/ruby-ci-image                                gcc-15               771cea939adf  6 days ago      3.18 GB
docker.io/library/gcc                                     15                   9a52dbcd9b63  2 weeks ago     1.57 GB

Maybe we could use gcc:15 but then we'd also need ruby inside, or change Makefile to no longer depend on ruby for SOEXT (or just fallback to .so if no ruby).

[eregon]

@kddnewton Could you check the commit message? It explains it.

To explain a bit more, the code does memset(breakpoints, 0, PM_STRPBRK_CACHE_SIZE); but that's at least theoretically invalid because PM_STRPBRK_CACHE_SIZE=16 and breakpoints = lex_mode.as.list.breakpoints and that's defined as uint8_t breakpoints[11]; or uint8_t breakpoints[7];.
It's an out of bound access, but maybe it doesn't cause any issue in practice due to alignment and/or the union, nevertheless it seems far safer to use uint8_t breakpoints[PM_STRPBRK_CACHE_SIZE]; (and fixes the GCC 9 error).

[eregon]

I switched to gcc images and it's quite a bit faster :) 6m49s