libc++: add NaCl and PNaCl support for std::random_device

Summary:
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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223068 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -111,6 +111,13 @@
 # endif
 #endif // __sun__
 
+#if defined(__native_client__)
+  // NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
+  // including accesses to the special files under /dev. C++11's
+  // std::random_device is instead exposed through a NaCl syscall.
+# define _LIBCPP_USING_NACL_RANDOM
+#endif // defined(__native_client__)
+
 #if !defined(_LIBCPP_LITTLE_ENDIAN) || !defined(_LIBCPP_BIG_ENDIAN)
 # include <endian.h>
 # if __BYTE_ORDER == __LITTLE_ENDIAN