elf: Fix tst-cpu-features-cpuinfo for KVM guests on some AMD systems [BZ #28704]

On KVM guests running on some AMD systems, the IBRS feature is reported
as a synthetic feature using the Intel feature, while the cpuinfo entry
keeps the same. Handle that by first checking the presence of the Intel
feature on AMD systems.

Fixes bug 28704.

(cherry picked from commit 94058f6c)