[libc++] Don't return uninitialized data from random_device::operator()

Make sure we appropriately retry calls to read if the return result is
less than what we asked for.

Additionally, check and handle IO errors: EINTR results in the read
operation getting restarted; other errors turn into exceptions.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210061 91177308-0d34-0410-b5e6-96231b3b80d8