x86: Install <bits/platform/x86.h> [BZ #27958]

1. Install <bits/platform/x86.h> for <sys/platform/x86.h> which includes
<bits/platform/x86.h>.
2. Rename HAS_CPU_FEATURE to CPU_FEATURE_PRESENT which checks if the
processor has the feature.
3. Rename CPU_FEATURE_USABLE to CPU_FEATURE_ACTIVE which checks if the
feature is active.  There may be other preconditions, like sufficient
stack space or further setup for AMX, which must be satisfied before the
feature can be used.

This fixes BZ #27958.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>