x86/cpuid: Calculate FEATURESET_NR_ENTRIES more helpfully

x86/cpuid: Calculate FEATURESET_NR_ENTRIES more helpfully

When adding new featureset words, it is convenient to split the work into
several patches.  However, GCC 12 spotted that the way we prefer to split the
work results in a real (transient) breakage whereby the policy <-> featureset
helpers perform out-of-bounds accesses on the featureset array.

Fix this by having gen-cpuid.py calculate FEATURESET_NR_ENTRIES from the
comments describing the word blocks, rather than from the XEN_CPUFEATURE()
with the greatest value.

For simplicty, require that the word blocks appear in order.  This can be
revisted if we find a good reason to have blocks out of order.

No functional change.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>