Add builtin folly::nextafter when using uclibc
Add builtin folly::nextafter when using uclibc

Summary:
When building using uclibc, std::nextafter doesn't exist. This is
similar case as for android, where std::nextafter doesn't exist.
Add additional condition to choose folly supplied versions of this function.

Reviewed By: yfeldblum

Differential Revision: D15291534

fbshipit-source-id: f6c9e213248f4e2a6e88e20578aeade46dc85d6b

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly