x86: debugctlmsr constants

This adds constant macros for a few of the bits in MSR_IA32_DEBUGCTLMSR.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>