x86, bts: detect size of DS fields

Impact: more robust DS feature enumeration

Detect the size of the pointer-type fields in the DS area
configuration via the DTES64 features rather than based on
the cpuid.

Rename a variable to denote that size to reflect that it only
covers the pointer-type fields.

Add more boot-time diagnostics giving the detected size and
the sizes of BTS and PEBS records.

Use the size of the BTS/PEBS record to indicate that the
respective feature is not available (if the record size is zero).

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
LKML-Reference: <20090313104218.A30096@sedona.ch.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>