libc++: support newlib's ctype

Summary: Newlib supports ctype differently from other platforms, this patch teaches libc++ about yet another platform that does ctype differently.

Reviewers: jroelofs

Subscribers: cfe-commits, danalbert, EricWF, jvoung, jfb, mclow.lists

Differential Revision: http://reviews.llvm.org/D7888

llvm-svn: 230557