kvm-unit-tests: test for rdseed/rdrand

kvm-unit-tests: test for rdseed/rdrand

The build fails when the host binutils isn't at least 2.23
(2.22.x introduced RDSEED).

The host toolchain is used for x86_64 target builds where
we need to do a 32bit build. Most other buildroot builds
are using a much newer binutils unless it's a external
older toolchain.

Fixes:
http://autobuild.buildroot.net/results/c39/c3987a3cbd2960b0ff50f872636bdfd8d1a9c820/

Upstream:
https://marc.info/?l=kvm&m=151580743523259&w=2

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>