[libc] Add implementations of nextafter[f|l] functions.

A differential fuzzer for these functions has also been added.
Along the way, a small correction has been done to the normal/subnormal
limits of x86 long double values.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D94109

GitOrigin-RevId: 7f7b0dc4e15fac5f91f8f6dcc7f91c9025f41ae0
21 files changed