Reorganize _LIBCPP_LOCALE__L_EXTENSIONS

Instead of checking _LIBCPP_LOCALE_L_EXTENSIONS all over, instead check it
once, and define the various *_l symbols once. The private redirector symbol
names are all prefixed with _libcpp_* so that they won't conflict with user
symbols, and so they won't conflict with future C library symbols. In
particular, glibc likes providing private symbols such as __locale_t, so we
should follow a different naming pattern (like _libcpp_*) to avoid problems
on that front.

Tested on Linux with glibc. Hoping for the best on OSX and the various BSDs.

http://reviews.llvm.org/D17456

llvm-svn: 263016