[Bug c/124699] [13 only] Expression using __builtin_ffsll() may not be considered constant

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124699

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 64088
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=64088&action=edit
testcase that shows issue

This testcase shows the issue better. Basically I changed the static assert so
it will not fail for GCC 14+ and cause a failure for 13-.