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

[JF Bastien]

> The purpose of the token is to permit different sources of randomness.
Seeing as how you don't allow anything other than
`nacl_secure_random_init`, perhaps you should just make the default token
be the empty string in the `<random>` header.

That would be a breaking change if we do decide to support other sources of
randomness. What we have now emulates the token it accepts (its POSIX
meaning), so it's the right thing IMO.

http://reviews.llvm.org/D6442