Attached patch adds __builtin_iseqsig() to the middle-end and C family front-ends.
Testing does not currently check whether the signaling part works, because with optimisation is actually does not (preexisting compiler bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106805)

Bootstrapped and regtested on x86_64-linux.
OK to commit?

(I’m not very skilled for middle-end hacking, so I’m sure there will be modifications to make.)

FX