Direct support for the x86 RDRAND instruction

From: "H. Peter Anvin" <hpa@linux.intel.com>

This is a proposed patchset to enable the new x86 RDRAND instruction,
labelled "Bull Mountain Technology" by Intel.  It is a different beast
than any other hardware random number generator that I have personally
encountered: it is not just a random number source, but contains a
high bandwidth random number generator, an AES cryptographic whitener,
and integrity monitoring all in hardware.

For technical documentation see:

http://software.intel.com/en-us/articles/download-the-lat...

This proposed patchset enables RDRAND bypass for current users of the
nonblocking random pool (that is, for /dev/urandom and its equvalent
in-kernel users) but not for the blocking pool (/dev/random).  This is
because RDRAND, although reseeded way more frequently than what is
practical to do in software, is technically a nonblocking source that
can behave as a PRNG.  It can be used as a source for randomness for
/dev/random, but that is not addressed by this patchset.