x86, bts: DS and BTS initialization

Impact: widen BTS/PEBS ptrace enablement to more CPU models

Move BTS initialisation out of an #ifdef CONFIG_X86_64 guard.

Assume core2 BTS and DS layout for future models of family 6 processors.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>