libm: Implement iseqsig()

[kfv]

Summary

C23 adds iseqsig(), the signaling counterpart of ==, which
raises FE_INVALID on any NaN rather than only on signaling
ones (as per §7.12.17.7). The helpers raise it explicitly
instead of relying on relational operators, which the
compiler may emit as quiet comparisons, and the macro
dispatches on float_t so that wider-evaluated arguments
are not narrowed (§F.10.14.1).