[PATCH] libc++: add NaCl and PNaCl support for std::random_device

[Marshall Clow]

I'd really prefer a different identifier other than `__native_client__`.
IMHO, that's too "generic".

Something that mentioned NaCl and/or PNaCl.
`_LIBCPP_USING_NACL_RANDOM`, maybe?

http://reviews.llvm.org/D6442