x86: make the x86 detection configurable with flags (internally)

Use flags to enable full discovery, caches only, or annotating cpuinfo only.
Enable them depending on whether caches or other objects being already present.
x86: export hwloc_x86_cpuid_discovery() to plugins

Make it accept a cpuid callback+data for Xen.
hwloc/plugins.h: protect cpuid functions with HWLOC_HAVE_X86_CPUID

But keep them enabled in the doc even when cpuid isn't available.

And make sure external plugins get HWLOC_HAVE_X86_CPUID from the public
autogen/config.h