This is a partial implementation of a fix for PR35672:

https://bugs.llvm.org/show_bug.cgi?id=35672

If this is on the right track, then I can add similar code for other transcendentals (exp2, log, log10, log2, pow).

Some questions:

- Do the finite calls need the double-leading underscores? I saw an existing test with __sqrt_finite, so I assume we want those, but I'm not sure if/how the regular calls acquire the underscores.
- Does this make sense for ISD::STRICT_FEXP (the strict version of the node)?
- Does the mathlib actually support the long double variants?