libc++: add NaCl and PNaCl support for std::random_device
The NaCl sandbox doesn't allow opening files under /dev, but it offers an API which provides the same capabilities. This is the same random device emulation that nacl_io performs for POSIX support, but nacl_io is an optional library so libc++ can't assume that device emulation will be performed. Note that NaCl only supports /dev/urandom, not /dev/random.
This patch also cleans up some of the preprocessor #endif, and fixes the test for Win32 (it accepts any token, and would therefore never throw regardless of the token provided).
Test Plan: ninja check-libcxx
Reviewers: dschuff, mclow.lists, danalbert
Subscribers: jfb, cfe-commits
Differential Revision: http://reviews.llvm.org/D6442