base/dispatch.c: Make `cpuid_feature_p' more easily extensible.

It turns out that Intel scatter feature flags throughout the various
CPUID leaves.  Change the interface for checking these flags so that we
can cover more ground without too much extra work.

  * Firstly, rename the function to `cpuid_feature_p' because it's only
    really useful for checking one feature at a time.

  * Secondly, make the first argument be a code indicating which
    particular `cpuid' output we're interested in; the second is still a
    mask used to check for the bit we're interested in.

Obviously this involves changing all of the callers too.