Revert "x86: check hwloc_have_cpuid() in the backend instantiate instead of late in the discovery"

This reverts commit 6a1cb59936460ff798970929f0bd52fa8da2cc00.

Don't require others caller of look_x86() (Xen soon) to manually check
for cpuid availability at runtime.
cpuid: rename cpuid into x86_cpuid everywhere

To avoid possible confusion if we add some sort of cpuid-related code for
other architectures such as ARM.
hwloc/plugins.h: move common PCI func to a dedicated section