[libc] Add implementations of nearbyint[f|l].

The implementation is exactly the same as rint* as even rint does not
raise any floating point exceptions currently. [Note that the standards
do not specify that floating point exceptions must be raised - they
leave it up to the implementation to choose to raise FE_INEXACT when
rounding non-integral values.]

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D94112