x86: cleanup C1E enabled detection

Rename the "MSR_K8_ENABLE_C1E" MSR to INT_PENDING_MSG, which is the
name in the data sheet as well. Move the C1E mask to the header file.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>