random: x86 driver: Prefer RDSEED over RDRAND when available

Per
https://software.intel.com/en-us/blogs/2012/11/17/the-difference-between-rdrand-and-rdseed
, RDRAND is a PRNG seeded from the same source as RDSEED. The source is
more suitable as PRNG seed material, so prefer it when the RDSEED intrinsic
is available (indicated in CPU feature bits).

(cherry picked from commit 2cb54a800c3a42cc256adcda0be98bd0af39096c)