[libc][math] Entry points and tests needed for some already-implemented math functions

[Flandini]

From discussion with @lntue, the functions nextafterf128, nexttowardf128, nearbyintf128, and scalbnf128, and nanf128 need entry points and tests added. nanf128 will also need some updates to strtonan to make it work for 128-bit.

I plan to work on these.