x86/oprofile: use 64 bit values in IBS functions

The IBS code internally uses 32 bit values (a low and a high value) to
represent a 64 bit value. This patch changes this and now 64 bit
values are used instead. 64 bit MSR functions can be used now.

No functional changes.

Signed-off-by: Robert Richter <robert.richter@amd.com>