Disable C standards under C99 from kernel build

[minsoochoo0122_proton.me]

Summary

Separated from D43254

Test Plan

make buildworld
make buildkernel

[imp]

Thanks for splitting this out.

[imp]

sys/conf/kern.mk

285

Oh wait a sec.... This changes c99 too...

Is c99 now an alias on all supported compilers? I'm pretty sure it is, but wanted to double check.

at least current versions of clang/gcc12 do... with gcc 12's reaction -std=c98 being the best:
gcc12: error: unrecognized command-line option '-std=c98'; did you mean '-std=c18'?

[emaste]

sys/conf/kern.mk

285

I think gcc12+ and llvm15+ are the relevant (if not supported) compilers and they support c99

[minsoochoo0122_proton.me]

sys/conf/kern.mk

285

gcc supports c99 since gcc 4.5 with -std=c99 (https://gcc.gnu.org/c99status.html)
clang had c99 option since its early releases (https://clang.llvm.org/c_status.html)

[imp]

I think this is ready now, so I'll land it.
I'm satisfied that old-school construct was for our old-school 4.2 gcc, which has aged out of support everywhere.